test: Fix popup test crash on exit

If the done signal is emitted by the common event handler, the window and all of its children have already been destroyed, so don't try to render with invalid renderer objects.

test/testpopup.c

@@ -188,6 +188,10 @@ static void loop(void)
         SDLTest_CommonEvent(state, &event, &done);
     }
 
+    if (done) {
+        return;
+    }
+
     /* Show the tooltip if the delay period has elapsed */
     if (SDL_GetTicks() > tooltip_timer) {
         if (tooltip.win == NULL) {