wan/hdlc_x25.c: fix a NULL dereference

WAN: fixes a NULL dereference in hdlc_x25.

Reported-by: Adrian Bunk <bunk@kernel.org&gt;.>
Signed-off-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>