media: mediatek: vcodec: Fix possible invalid memory access for decoder

The vpu maybe null pointer or unreasonable value when scp crash, need to
validate that the vpu pointer and the vpu instance within this context is
valid in case of leading to kernel reboot.

Fixes: 590577a4

 ("[media] vcodec: mediatek: Add Mediatek V4L2 Video Decoder Driver")
Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reported-by: Steve Cho <stevecho@google.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>