isdn: isdnloop: fix pointer dereference bug

clang has spotted an ancient code bug and warns about it with:

drivers/isdn/isdnloop/isdnloop.c:573:12: error: address of array 'card->rcard' will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]

This is an array of pointers, so we should check if a specific
pointer exists in the array before using it, not whether the
array itself exists.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/isdnloop/isdnloop.c

@@ -570,7 +570,7 @@ isdnloop_atimeout(isdnloop_card *card, int ch)
 	char buf[60];
 
 	spin_lock_irqsave(&card->isdnloop_lock, flags);
-	if (card->rcard) {
+	if (card->rcard[ch]) {
 		isdnloop_fake(card->rcard[ch], "DDIS_I", card->rch[ch] + 1);
 		card->rcard[ch]->rcard[card->rch[ch]] = NULL;
 		card->rcard[ch] = NULL;