fix a possible memory leak in SDL_vasprintf()

(cherry picked from commit 1a83bf2399c29f3028477225067bf6a106068f08)

src/stdlib/SDL_string.c

@@ -1989,6 +1989,7 @@ int SDL_vasprintf(char **strp, const char *fmt, va_list ap)
 
         /* Check error code */
         if (retval < 0) {
+            SDL_free(p);
             return retval;
         }
 
@@ -2010,5 +2011,3 @@ int SDL_vasprintf(char **strp, const char *fmt, va_list ap)
         }
     }
 }
-
-/* vi: set ts=4 sw=4 expandtab: */