Add STBSP__ASAN for msvc compiler

[septag]

[Condzi]

I also had an issue with ASAN and the trick stbsp uses for finding '\0'. However, I found out that I also need to modify STBSP_PUBLICDEC / PUBLICDEF macros by adding STBSP__ASAN to them, like so:

#ifdef STB_SPRINTF_STATIC
#define STBSP__PUBLICDEC static STBSP__ASAN
#define STBSP__PUBLICDEF static STBSP__ASAN
#else
#ifdef __cplusplus
#define STBSP__PUBLICDEC extern "C" STBSP__ASAN
#define STBSP__PUBLICDEF extern "C" STBSP__ASAN
#else
#define STBSP__PUBLICDEC extern STBSP__ASAN
#define STBSP__PUBLICDEF STBSP__ASAN
#endif
#endif

I'm not sure why they aren't there in the first place. Without this added, my code still was giving me global-buffer-overflow.

[Manuzor]

I came here because I ran into global-buffer-overflow as well, even with the fixes in this PR, but the final missing piece was adding STBSP__ASAN to the public definitions, so thanks @Condzi for that info!

By the way, I think this PR is a duplicate of #1350 although it contains an explicit MSVC version check.