V4L/DVB (8955): bttv: Prevent NULL pointer dereference in radio_open

Fix the following crash in the bttv driver:

BUG: unable to handle kernel NULL pointer dereference at 000000000000036c
IP: [<ffffffffa037860a>] radio_open+0x3a/0x170 [bttv]

This happens because radio_open assumes that all present bttv devices
have a radio function. If a bttv device without radio and one with
radio are installed on the same system, and the one without radio is
registered first, then radio_open checks for the radio device number
of a bttv device that has no radio function, and this breaks. All we
have to do to fix it is to skip bttv devices without a radio function.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/bt8xx/bttv-driver.c

@@ -3431,7 +3431,7 @@ static int radio_open(struct inode *inode, struct file *file)
 	dprintk("bttv: open minor=%d\n",minor);
 
 	for (i = 0; i < bttv_num; i++) {
-		if (bttvs[i].radio_dev->minor == minor) {
+		if (bttvs[i].radio_dev && bttvs[i].radio_dev->minor == minor) {
 			btv = &bttvs[i];
 			break;
 		}