-D_WINE_MSVCRT does not check for fmt==NULL

with *printf_s and *scanf_s.
e.g. https://github.com/wine-mirror/wine/blob/master/dlls/msvcrt/file.c#L5378

msvcrt doesn't even check for src==NULL.

there's no mingw cross-specific define, when running under WINE
so we need to add one. Just used for testing.

now all TEST_MSVCRT tests pass under mingw native and wine-2 (darwin).