snake: check allocated memory before use

examples/game/01-snake/snake.c

@@ -310,6 +310,9 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
     }
 
     AppState *as = SDL_calloc(1, sizeof(AppState));
+    if (!as) {
+        return SDL_APP_FAILURE;
+    }
 
     *appstate = as;