Fix possible memory overrun issue in the isdn ioctl code.

Found by ADLAB <adlab@venustech.com.cn>

Signed-off-by: Karsten Keil <kkeil@suse.de>
Cc: ADLAB <adlab@venustech.com.cn>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>