media: drivers/media/v4l2-core/v4l2-h264 : add detection of null pointers

When the pointer variable is judged to be null, null is returned
directly.

[hverkuil: fix two checkpatch warnings]

Signed-off-by: Dong Chuanjian <chuanjian@nfschina.com>
Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Fixes: d3f756ad ("media: v4l2: Trace calculated p/b0/b1 initial reflist")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>