stdlib: Fixed crash on SDL_snprintf("%s", NULL).

Like other C runtimes, it should probably produce the string "(null)".

This bug probably only affected Windows, as most platforms use their standard
C runtime's snprintf().

src/stdlib/SDL_string.c

@@ -1313,6 +1313,10 @@ SDL_PrintString(char *text, size_t maxlen, SDL_FormatInfo *info, const char *str
     size_t length = 0;
     size_t slen;
 
+    if (string == NULL) {
+        string = "(null)";
+    }
+
     if (info && info->width && (size_t)info->width > SDL_strlen(string)) {
         char fill = info->pad_zeroes ? '0' : ' ';
         size_t width = info->width - SDL_strlen(string);