[media] dvb core: only start media entity if not NULL

The logic there tries to start the media entity even if it
doesn't exist, causing this bug:

	[  314.356162] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
	[  314.356202] IP: [<ffffffffa02ef74c>] media_entity_pipeline_start+0x1c/0x390 [media]

Reported-by: Gert-Jan van der Stroom <gjstroom@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>