Safety fix: clear "_this->current_glwin" when destroying the window

(cherry picked from commit 0c048d98af449f1bc540e256c90eb792f1effff4)

src/video/SDL_video.c

@@ -3272,6 +3272,10 @@ void SDL_DestroyWindow(SDL_Window *window)
         _this->grabbed_window = NULL; /* ungrabbing input. */
     }
 
+    if (_this->current_glwin == window) {
+        _this->current_glwin = NULL;
+    }
+
     /* Now invalidate magic */
     window->magic = NULL;