V4L/DVB (6188): Avoid a NULL pointer dereference during mpeg_open()

Bug: With a hardware encoder board installed as cx88[1] and a
non-encoder boards installed as cx88[0], an OOPS is generated
during cx8802_get_device() called from mpeg_open().

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/cx88/cx88-mpeg.c

@@ -580,7 +580,7 @@ struct cx8802_dev * cx8802_get_device(struct inode *inode)
 
 	list_for_each(list,&cx8802_devlist) {
 		h = list_entry(list, struct cx8802_dev, devlist);
-		if (h->mpeg_dev->minor == minor)
+		if (h->mpeg_dev && h->mpeg_dev->minor == minor)
 			return h;
 	}