switch from fopen to fopen_s on later MSVCs

Proper pointer check before deleting

minor warn correction

Explicit CDECL needed for qsort comparison function

QSORT only takes CDECL, but project settings can define different defaults for calling conventions.

gcc warning: unused parameter removed for macro STBTT_free

corrected annoying warnings about 'missing initializer'. The memset
version is not better, but gcc like it more.

to avoid missing bugs like the recent TGA bug in the future. Doesn't check the
*results*, but tests that it doesn't crash or fail.