V4L/DVB (6328): ivtv: fix NULL dereference

We shouldn't dereference "itv" when we know it's NULL...

Spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>