LoadIniSettingsFromDisk: fix out-of-bounds read when file is empty

[quantum5]

When the ini file has zero length, LoadIniSettingsFromDisk passes 0 as ini_size to LoadIniSettingsFromMemory. However, LoadIniSettingsFromMemory understands it as "detect ini size" and promptly runs strlen on the buffer. Here, the buffer is zero-sized, so strlen reads out-of-bounds, trying to find the null terminator in vain.

The solution is to just not call LoadIniSettingsFromMemory when there is nothing to parse anyway.

Example ASan error:

    #0 0x7f6cfc6f4b10 in __interceptor_strlen ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:370
    #1 0x55f13e3fcb18 in ImGui::LoadIniSettingsFromMemory(char const*, unsigned long) /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:11606
    #2 0x55f13e3fca2c in ImGui::LoadIniSettingsFromDisk(char const*) /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:11591
    #3 0x55f13e3fc02f in UpdateSettings /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:11494
    #4 0x55f13e3a14ad in ImGui::NewFrame() /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:4237
    #5 0x55f13e35be4b in igNewFrame (/home/quantum/build/LookingGlass/client/build/looking-glass-client+0x19ae4b)
    #6 0x55f13e2c601b in app_renderOverlay /home/quantum/build/LookingGlass/client/src/app.c:862
    #7 0x55f13e5c01df in egl_render /home/quantum/build/LookingGlass/client/renderers/EGL/egl.c:1099
    #8 0x55f13e29622a in renderThread /home/quantum/build/LookingGlass/client/src/main.c:291
    #9 0x55f13e623261 in threadWrapper /home/quantum/build/LookingGlass/common/src/platform/linux/thread.c:40
    #10 0x7f6cfbe75ea6 in start_thread nptl/pthread_create.c:477
    #11 0x7f6cfba24dee in __clone (/lib/x86_64-linux-gnu/libc.so.6+0xfddee)

0x6020000a8631 is located 0 bytes to the right of 1-byte region [0x6020000a8630,0x6020000a8631)
allocated by thread T4 here:
    #0 0x7f6cfc761e8f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x55f13e37d3c7 in MallocWrapper /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:1019
    #2 0x55f13e399e0e in ImGui::MemAlloc(unsigned long) /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:3603
    #3 0x55f13e3871de in ImFileLoadToMemory(char const*, char const*, unsigned long*, int) /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:1802
    #4 0x55f13e3fc9e8 in ImGui::LoadIniSettingsFromDisk(char const*) /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:11588
    #5 0x55f13e3fc02f in UpdateSettings /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:11494
    #6 0x55f13e3a14ad in ImGui::NewFrame() /home/quantum/build/LookingGlass/repos/cimgui/imgui/imgui.cpp:4237
    #7 0x55f13e35be4b in igNewFrame (/home/quantum/build/LookingGlass/client/build/looking-glass-client+0x19ae4b)
    #8 0x55f13e2c601b in app_renderOverlay /home/quantum/build/LookingGlass/client/src/app.c:862
    #9 0x55f13e5c01df in egl_render /home/quantum/build/LookingGlass/client/renderers/EGL/egl.c:1099
    #10 0x55f13e29622a in renderThread /home/quantum/build/LookingGlass/client/src/main.c:291
    #11 0x55f13e623261 in threadWrapper /home/quantum/build/LookingGlass/common/src/platform/linux/thread.c:40
    #12 0x7f6cfbe75ea6 in start_thread nptl/pthread_create.c:477

[ocornut]

Merged, thank you very much !