[media] saa7164: Check if dev is NULL before dereferencing it

As reported by smatch:
	drivers/media/pci/saa7164/saa7164-core.c:631 saa7164_irq() warn: variable dereferenced before check 'dev' (see line 621)

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>