Only check if we need to do a SIGMA based tone generation if we have a card. (issue #10179 reported by mikowhy)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@74515 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_phone.c

@@ -1044,7 +1044,7 @@ static void *do_monitor(void *data)
 		ast_mutex_unlock(&iflock);
 
 		/* Wait indefinitely for something to happen */
-		if (dotone && i->mode != MODE_SIGMA) {
+		if (dotone && i && i->mode != MODE_SIGMA) {
 			/* If we're ready to recycle the time, set it to 30 ms */
 			tonepos += 240;
 			if (tonepos >= sizeof(DialTone))