mac80211: fix bug in ieee80211_build_probe_req

ieee80211_probereq_get() can return NULL in
which case we should clean up & return NULL
in ieee80211_build_probe_req() as well.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>