media: vicodec: Handle the case that the reference buffer is NULL

In the stateless decoder the reference buffer is null if the
frame is an I-frame (flagged with FWHT_FL_I_FRAME).
Make sure not to dereference it in that case.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>