This is already done in WndProc, but the flag was not set if we exited
event loop and were processing mpv dispatch messages in fallback loop.
This fixes the case when Window is destroyed externally or otherwise
event loop fails for whatever reason. Calling vo_w32_uninit does call
DestroyWindow() through dispatch, which in turn calls WndProc, so we
need to guard recursive call.
fe546c1841