Check nullptr before calling the windows message hook for WM_ENTERSIZEMOVE and WM_ENTERMENULOOP

src/video/windows/SDL_windowsevents.c

@@ -1721,9 +1721,11 @@ LRESULT CALLBACK WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lPara
     case WM_ENTERSIZEMOVE:
     case WM_ENTERMENULOOP:
     {
+        if (g_WindowsMessageHook) {
             if (!DispatchModalLoopMessageHook(&hwnd, &msg, &wParam, &lParam)) {
                 return 0;
             }
+        }
 
         ++data->in_modal_loop;
         if (data->in_modal_loop == 1) {