commit 588afcc1 upstream.

This fixes the crash reported in:
http://seclists.org/bugtraq/2015/Oct/35
The interface number needs a sanity check.
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Cc: Moritz Muehlenhoff <jmm@inutil.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>