FROMLIST: media: mtk-vcodec: venc: make sure buffer exists in list before removing

It is possible that empty_flush_buf is removed in mtk_venc_worker() and
then again in vb2ops_venc_stop_streaming(). However, there's no empty
list check in v4l2_m2m_buf_remove_by_buf(). Double remove causes a
kernel crash.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
[acourbot: fix commit log a bit]
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>