media: mediatek: vcodec: Fix getting NULL pointer for dst buffer

The driver may can't get v4l2 buffer when lat or core decode timeout,
will lead to crash when call v4l2_m2m_buf_done to set dst buffer
(NULL pointer) done.

Fixes: 7b182b8d ("media: mediatek: vcodec: Refactor get and put capture buffer flow")
Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>