If we didn't create win32 window, restore its event procedure on destruction.

src/video/windows/SDL_windowswindow.c

@@ -636,6 +636,15 @@ WIN_DestroyWindow(_THIS, SDL_Window * window)
         ReleaseDC(data->hwnd, data->hdc);
         if (data->created) {
             DestroyWindow(data->hwnd);
+        } else {
+            /* Restore any original event handler... */
+            if (data->wndproc != NULL) {
+#ifdef GWLP_WNDPROC
+                SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR) data->wndproc);
+#else
+                SetWindowLong(hwnd, GWL_WNDPROC, (LONG_PTR) data->wndproc);
+#endif
+            }
         }
         SDL_free(data);
     }