Fixed bug 2596 - SDL_SetError fails on on NULL on systems with vsnprintf

sfalexrog

On systems with vsnprintf call SDL_SetError fails when passed a NULL as an argument. SDL's implementation checks for NULL (as seen in the commit: https://hg.libsdl.org/SDL/rev/835403a6aec8), but system implementation may crash.

src/stdlib/SDL_string.c

@@ -1276,6 +1276,9 @@ SDL_snprintf(SDL_OUT_Z_CAP(maxlen) char *text, size_t maxlen, SDL_PRINTF_FORMAT_
 #ifdef HAVE_VSNPRINTF
 int SDL_vsnprintf(SDL_OUT_Z_CAP(maxlen) char *text, size_t maxlen, const char *fmt, va_list ap)
 {
+    if (!fmt) {
+        fmt = "";
+    }
     return vsnprintf(text, maxlen, fmt, ap);
 }
 #else