mac80211: Fix potential null pointer dereferencing

The patch "{nl,cfg,mac}80211: Implement RSSI threshold for mesh peering"
has a potential null pointer dereferencing problem. Thanks to Dan Carpenter
for pointing out. This patch will fix the issue.

Signed-off-by: Ashok Nagarajan <ashok@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>