media: media/saa7146: fix incorrect assertion in saa7146_buffer_finish

In saa7146_buffer_finish, the code for q->curr to be NULL is
already present and asserting for NULL is unnecessary. This patch
elimiates such a check.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>