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>