fix ASAN error in Vec2<float>::Length()

[oltolm]

I compiled ppsspp with Clang on Windows and ASAN and found this bug.

=================================================================
==4456==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x007c60dfb3ff at pc 0x7ff71c8ec86f bp 0x007c60dfafe0 sp 0x007c60dfb028
READ of size 16 at 0x007c60dfb3ff thread T6
    #0 0x7ff71c8ec86e in Math3D::Vec2<float>::Length() const C:/src/ppsspp/GPU/Math3D.cpp:29:14
    #1 0x7ff71c8ed431 in Math3D::Vec2<float>::Normalized() const C:/src/ppsspp/GPU/Math3D.cpp:66:19
    #2 0x7ff71e7303b0 in SoftwareTransform::ExpandLines(int, int&, unsigned short*&, TransformedVertex const*, TransformedVertex*, int&, bool) C:/src/ppsspp/GPU/Common/SoftwareTransformCommon.cpp:778:50
    #3 0x7ff71e72bc73 in SoftwareTransform::BuildDrawingParams(int, int, unsigned int, unsigned short*&, int&, SoftwareTransformResult*) C:/src/ppsspp/GPU/Common/SoftwareTransformCommon.cpp:588:3
    #4 0x7ff71e1092e9 in DrawEngineD3D11::DoFlush() C:/src/ppsspp/GPU/D3D11/DrawEngineD3D11.cpp:429:16
    #5 0x7ff71e113baf in DrawEngineD3D11::Flush() C:/src/ppsspp/GPU/D3D11/DrawEngineD3D11.h:84:3
    #6 0x7ff71e110ccf in DrawEngineD3D11::DispatchFlush() C:/src/ppsspp/GPU/D3D11/DrawEngineD3D11.h:96:3
    #7 0x7ff71dee8a70 in GPUCommonHW::FastRunLoop(DisplayList&) C:/src/ppsspp/GPU/GPUCommonHW.cpp:844:24
    #8 0x7ff71dffd1c6 in GPUCommon::InterpretList(DisplayList&) C:/src/ppsspp/GPU/GPUCommon.cpp:675:4
    #9 0x7ff71dff9c57 in GPUCommon::ProcessDLQueue() C:/src/ppsspp/GPU/GPUCommon.cpp:845:8
    #10 0x7ff71dffad15 in GPUCommon::UpdateStall(int, unsigned int) C:/src/ppsspp/GPU/GPUCommon.cpp:506:2
    #11 0x7ff71c4f1122 in sceGeListUpdateStallAddr(unsigned int, unsigned int) C:/src/ppsspp/Core/HLE/sceGe.cpp:380:14
    #12 0x7ff71c4f0290 in void WrapI_UU<&sceGeListUpdateStallAddr(unsigned int, unsigned int)>() C:/src/ppsspp/Core/HLE/FunctionWrappers.h:200:15
    #13 0x7ff71b69136c in CallSyscallWithoutFlags(HLEFunction const*) C:/src/ppsspp/Core/HLE/HLE.cpp:659:2
    #14 0x7ff71980d50f  (<unknown module>)

Address 0x007c60dfb3ff is located in stack of thread T5 at offset 95 in frame
    #0 0x7ff71e72f68f in SoftwareTransform::ExpandLines(int, int&, unsigned short*&, TransformedVertex const*, TransformedVertex*, int&, bool) C:/src/ppsspp/GPU/Common/SoftwareTransformCommon.cpp:741

  This frame has 8 object(s):
    [32, 36) 'horizontal' (line 775)
    [48, 56) 'addWidth' (line 778)
    [80, 88) 'ref.tmp' (line 778)
    [112, 116) 'ref.tmp36' (line 778) <== Memory access at offset 95 underflows this variable
    [128, 132) 'horizontal156' (line 823)
    [144, 152) 'addWidth172' (line 825)
    [176, 184) 'ref.tmp173' (line 825)
    [208, 212) 'ref.tmp174' (line 825)
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp, SEH and C++ exceptions *are* supported)
Thread T5 created by T0 here:
    #0 0x7ffcc0cf3576 in CreateThread (C:\msys64\clang64\bin\libclang_rt.asan_dynamic-x86_64.dll+0x180053576)
    #1 0x7ffc95a4e755  (<unknown module>)
    #2 0x7ffc93598df1  (<unknown module>)
    #3 0x7ffc935a94ed  (<unknown module>)
    #4 0x7ffc935a9c68  (<unknown module>)
    #5 0x7ffc93497173  (<unknown module>)
    #6 0x7ffc93497500  (<unknown module>)
    #7 0x7ffc934966bd  (<unknown module>)
    #8 0x7ffc93495f13  (<unknown module>)
    #9 0x7ffc934896ac  (<unknown module>)
    #10 0x7ffc934e63ed  (<unknown module>)
    #11 0x7ffc934f2298  (<unknown module>)
    #12 0x7ffc934ffdd2  (<unknown module>)
    #13 0x7ffc934ff924  (<unknown module>)
    #14 0x7ffc9350183a  (<unknown module>)
    #15 0x7ffcb86ad2ed  (<unknown module>)
    #16 0x7ffc9357b84f  (<unknown module>)
    #17 0x7ffcb8695014  (<unknown module>)
    #18 0x7ffcb86b3653  (<unknown module>)
    #19 0x7ff71bdcd1e5 in VulkanMayBeAvailable() C:/src/ppsspp/Common/GPU/Vulkan/VulkanLoader.cpp:447:8
    #20 0x7ff71b41926d in Config::IsBackendEnabled(GPUBackend, bool) C:/src/ppsspp/Core/Config.cpp:537:41
    #21 0x7ff71b3a11b3 in MainWindow::UpdateMenus(bool) C:/src/ppsspp/Windows/MainWindowMenu.cpp:1162:31
    #22 0x7ff71b389b31 in MainWindow::Show(HINSTANCE__*) C:/src/ppsspp/Windows/MainWindow.cpp:521:3
    #23 0x7ff71b3d85d1 in wWinMain C:/src/ppsspp/Windows/main.cpp:991:2
    #24 0x7ff71c16d625 in wmain C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crtexewin.c:67:10
    #25 0x7ff71a8e1318 in __tmainCRTStartup C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crtexe.c:267:15
    #26 0x7ff71a8e1155 in .l_startw C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crtexe.c:157:9
    #27 0x7ffd64fa7343  (C:\WINDOWS\System32\KERNEL32.DLL+0x180017343)
    #28 0x7ffd650e26b0  (C:\WINDOWS\SYSTEM32\ntdll.dll+0x1800526b0)

SUMMARY: AddressSanitizer: stack-buffer-overflow C:/src/ppsspp/GPU/Math3D.cpp:29:14 in Math3D::Vec2<float>::Length() const
Shadow bytes around the buggy address:
  0x007c60dfb100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x007c60dfb180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x007c60dfb200: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 f2 f2 f2
  0x007c60dfb280: 04 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x007c60dfb300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x007c60dfb380: 00 00 00 00 f1 f1 f1 f1 04 f2 00 f2 f2 f2 00[f2]
  0x007c60dfb400: f2 f2 04 f2 f8 f2 f8 f2 f2 f2 f8 f2 f2 f2 f8 f3
  0x007c60dfb480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x007c60dfb500: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x007c60dfb580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x007c60dfb600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
Thread T6 created by T0 here:
    #0 0x7ffcc0cf3576 in CreateThread (C:\msys64\clang64\bin\libclang_rt.asan_dynamic-x86_64.dll+0x180053576)
    #1 0x7ffd63011896  (C:\WINDOWS\System32\ucrtbase.dll+0x180021896)
    #2 0x7ffd3562a785 in std::__1::__libcpp_thread_create(void**, void* (*)(void*), void*) (C:\msys64\clang64\bin\libc++.dll+0x18009a785)
    #3 0x7ff71b31ebfb in std::__1::thread::thread<void (*)(), void>(void (*&&)()) C:/msys64/clang64/include/c++/v1/__thread/thread.h:248:16
    #4 0x7ff71b31bcc2 in MainThread_Start(bool) C:/src/ppsspp/Windows/EmuThread.cpp:64:15
    #5 0x7ff71b3d890e in wWinMain C:/src/ppsspp/Windows/main.cpp:1018:2
    #6 0x7ff71c16d625 in wmain C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crtexewin.c:67:10
    #7 0x7ff71a8e1318 in __tmainCRTStartup C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crtexe.c:267:15
    #8 0x7ff71a8e1155 in .l_startw C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crtexe.c:157:9
    #9 0x7ffd64fa7343  (C:\WINDOWS\System32\KERNEL32.DLL+0x180017343)
    #10 0x7ffd650e26b0  (C:\WINDOWS\SYSTEM32\ntdll.dll+0x1800526b0)

==4456==ABORTING

[hrydgard]

Yeah that's a good fix (though in practice, it's not gonna matter, but good to be asan-clean).

[fp64]

// Doubt this is worth it for a vec2 :/

FWIW, while this might be slightly faster for random access, clang autovectorizes the fallback version to something 3x faster for sequential access over a bunch of Vec2's:
https://godbolt.org/z/MxW7j8zKn

Seems unlikely to matter either way.