Fixed infinite loop in SDL_vsnprintf() if the format string is too large for the output buffer

Fixes https://github.com/libsdl-org/SDL/issues/4940

src/stdlib/SDL_string.c

@@ -1887,8 +1887,9 @@ SDL_vsnprintf(SDL_OUT_Z_CAP(maxlen) char *text, size_t maxlen, const char *fmt,
             }
         } else {
             if (length < maxlen) {
-                text[length] = *fmt++;
+                text[length] = *fmt;
             }
+            ++fmt;
             ++length;
         }
     }