libsepol/cil: always destroy the lexer state

It was found in google/oss-fuzz#4790:
```
Invalid token '' at line 2 of fuzz
	NEW_FUNC[1/2]: 0x67fff0 in yy_get_previous_state /src/selinux/libsepol/src/../cil/src/cil_lexer.c:1143
	NEW_FUNC[2/2]: 0x6803e0 in yy_try_NUL_trans /src/selinux/libsepol/src/../cil/src/cil_lexer.c:1176
=================================================================
==12==ERROR: AddressSanitizer: heap-use-after-free on address 0x602000007992 at pc 0x000000681800 bp 0x7ffccddee530 sp 0x7ffccddee528
WRITE of size 1 at 0x602000007992 thread T0
SCARINESS: 41 (1-byte-write-heap-use-after-free)
    #0 0x6817ff in cil_yy_switch_to_buffer /src/selinux/libsepol/src/../cil/src/cil_lexer.c:1315:17
    #1 0x6820cc in cil_yy_scan_buffer /src/selinux/libsepol/src/../cil/src/cil_lexer.c:1571:2
    #2 0x682662 in cil_lexer_setup /src/selinux/libsepol/src/../cil/src/cil_lexer.l:73:6
    #3 0x5cf2ae in cil_parser /src/selinux/libsepol/src/../cil/src/cil_parser.c:220:2
    #4 0x56d5e2 in cil_add_file /src/selinux/libsepol/src/../cil/src/cil.c:514:7
    #5 0x556e91 in LLVMFuzzerTestOneInput /src/secilc-fuzzer.c:434:7
    #6 0x459ab1 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:599:15
    #7 0x45a755 in fuzzer::Fuzzer::TryDetectingAMemoryLeak(unsigned char const*, unsigned long, bool) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:675:3
    #8 0x45acd9 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:747:5
    #9 0x45b875 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:883:5
    #10 0x4499fb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:906:6
    #11 0x473a32 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #12 0x7f982296d83f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2083f)
    #13 0x41e758 in _start (/out/secilc-fuzzer+0x41e758)

DEDUP_TOKEN: cil_yy_switch_to_buffer--cil_yy_scan_buffer--cil_lexer_setup
0x602000007992 is located 2 bytes inside of 4-byte region [0x602000007990,0x602000007994)
freed by thread T0 here:
    #0 0x521ef2 in free /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:127:3
    #1 0x56d630 in cil_add_file /src/selinux/libsepol/src/../cil/src/cil.c:526:2
    #2 0x556e91 in LLVMFuzzerTestOneInput /src/secilc-fuzzer.c:434:7
    #3 0x459ab1 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:599:15
    #4 0x458fba in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:505:3
    #5 0x45acc7 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:745:19
    #6 0x45b875 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:883:5
    #7 0x4499fb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:906:6
    #8 0x473a32 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #9 0x7f982296d83f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2083f)

DEDUP_TOKEN: free--cil_add_file--LLVMFuzzerTestOneInput
previously allocated by thread T0 here:
    #0 0x52215d in malloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:145:3
    #1 0x5cecb8 in cil_malloc /src/selinux/libsepol/src/../cil/src/cil_mem.c:39:14
    #2 0x56d584 in cil_add_file /src/selinux/libsepol/src/../cil/src/cil.c:510:11
    #3 0x556e91 in LLVMFuzzerTestOneInput /src/secilc-fuzzer.c:434:7
    #4 0x459ab1 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:599:15
    #5 0x458fba in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:505:3
    #6 0x45acc7 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:745:19
    #7 0x45b875 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:883:5
    #8 0x4499fb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:906:6
    #9 0x473a32 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #10 0x7f982296d83f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2083f)

DEDUP_TOKEN: malloc--cil_malloc--cil_add_file
SUMMARY: AddressSanitizer: heap-use-after-free /src/selinux/libsepol/src/../cil/src/cil_lexer.c:1315:17 in cil_yy_switch_to_buffer
Shadow bytes around the buggy address:
  0x0c047fff8ee0: fa fa fd fa fa fa fd fd fa fa fd fa fa fa fd fd
  0x0c047fff8ef0: fa fa fd fa fa fa fd fd fa fa fd fa fa fa fd fd
  0x0c047fff8f00: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fd
  0x0c047fff8f10: fa fa fd fa fa fa fd fd fa fa fd fa fa fa fd fd
  0x0c047fff8f20: fa fa fd fa fa fa fd fd fa fa fd fa fa fa fd fa
=>0x0c047fff8f30: fa fa[fd]fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fff8f40: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fff8f50: fa fa fd fa fa fa fd fd fa fa fd fa fa fa fd fa
  0x0c047fff8f60: fa fa fd fd fa fa fd fa fa fa fd fd fa fa fd fa
  0x0c047fff8f70: fa fa 00 00 fa fa 02 fa fa fa 02 fa fa fa 00 fa
  0x0c047fff8f80: fa fa 03 fa fa fa 00 fa fa fa 03 fa fa fa 00 fa
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
  Shadow gap:              cc
==12==ABORTING
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>
Acked-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsepol/cil/src/cil_parser.c

@@ -310,6 +310,7 @@ int cil_parser(char *_path, char *buffer, uint32_t size, struct cil_tree **parse
 	while (!cil_stack_is_empty(stack)) {
 		pop_hll_info(stack, &hll_lineno, &hll_expand);
 	}
+	cil_lexer_destroy();
 	cil_stack_destroy(&stack);
 
 	return SEPOL_ERR;