[media] go7007: don't use vb before test if it is not NULL

As reported by smatch:
	drivers/media/usb/go7007/go7007-driver.c:452 frame_boundary() warn: variable dereferenced before check 'vb' (see line 449)

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>