media: mediatek: vcodec: fix h264 cavlc bitstream fail

Some cavlc bistream will decode fail when the frame size is less than
20 bytes. Need to add pending data at the end of the bitstream.

For the minimum size of mapped memory is 256 bytes(16x16), adding four
bytes data won't lead to access unknown virtual memory.

Fixes: 59fba9ee ("media: mediatek: vcodec: support stateless H.264 decoding for mt8192")
Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>