Fixes #2417, memory leak in SDL_gamecontroller.c (thanks Leonardo!)

src/joystick/SDL_gamecontroller.c

@@ -1223,6 +1223,7 @@ SDL_GameControllerQuit(void)
         pControllerMap = s_pSupportedControllers;
         s_pSupportedControllers = s_pSupportedControllers->next;
         SDL_free( pControllerMap->name );
+        SDL_free( pControllerMap->mapping );
         SDL_free( pControllerMap );
     }