isdn: guard against a potential NULL pointer dereference in old_capi_manufacturer()

In drivers/isdn/capi/kcapi.c::old_capi_manufacturer(), if the call to
get_capi_ctr_by_nr(ldef.contr); in line 823 returns NULL, then we'll be
dereferencing a NULL pointer in the very next line.

(Found by Coverity checker as bug #402)

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/isdn/capi/kcapi.c

@@ -821,6 +821,8 @@ static int old_capi_manufacturer(unsigned int cmd, void __user *data)
 				return -EFAULT;
 		}
 		card = get_capi_ctr_by_nr(ldef.contr);
+		if (!card)
+			return -EINVAL;
 		card = capi_ctr_get(card);
 		if (!card)
 			return -ESRCH;