make sure SDL_vsnprintf() nul terminates if it is using _vsnprintf

The change makes sure that SDL_vsnprintf() nul terminates if it is
using _vsnprintf() for the job.

I made this patch for Watcom, whose _vsnprintf() doesn't guarantee
nul termination.  The preprocessor check can be extended to windows
in general too, if required.

Closes bug #3769.
1 file changed