[media] media: fix null pointer dereference in v4l_vb2q_enable_media_source()

Fix the null pointer dereference in v4l_vb2q_enable_media_source().
DVB only drivers don't have valid struct v4l2_fh pointer.

[ 548.443272] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000010
[ 548.452036] IP: [<ffffffffc020ffc9>]
v4l_vb2q_enable_media_source+0x9/0x50 [videodev]
[ 548.460792] PGD b820e067 PUD bb3df067 PMD 0
[ 548.465582] Oops: 0000 [#1] SMP

Signed-off-by: Shuah Khan <[e-mail]>
Reported-by: Olli Salonen <[e-mail]>
Tested-by: Olli Salonen <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/v4l2-core/v4l2-mc.c

···
 {
 	struct v4l2_fh *fh = q->owner;
 
-	return v4l_enable_media_source(fh->vdev);
+	if (fh && fh->vdev)
+		return v4l_enable_media_source(fh->vdev);
+	return 0;
 }
 EXPORT_SYMBOL_GPL(v4l_vb2q_enable_media_source);