security: avoid calling a NULL function pointer in drivers/video/tvaudio.c

NULL function pointers are very bad security wise. This one got caught by
kerneloops.org quite a few times, so it's happening in the field....

Fix is simple, check the function pointer for NULL, like 6 other places
in the same function are already doing.

Signed-off-by: Arjan van de Ven <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/media/video/tvaudio.c

···
 		break;
 	case VIDIOC_S_FREQUENCY:
 		chip->mode = 0; /* automatic */
-		if (desc->checkmode) {
+		if (desc->checkmode && desc->setmode) {
 			desc->setmode(chip,V4L2_TUNER_MODE_MONO);
 			if (chip->prevmode != V4L2_TUNER_MODE_MONO)
 				chip->prevmode = -1; /* reset previous mode */