[media] ivtv: steal could be NULL

ivtv_flush_queues() calls ivtv_queue_move() with steal == NULL.
However, part of the code assumes that steal could be not null, as
pointed by smatch:
	drivers/media/pci/ivtv/ivtv-queue.c:145 ivtv_queue_move() error: we previously assumed 'steal' could be null (see line 138)

This has the potential of causing an OOPS when the queue is
flushed.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>