mac80211: Fix scan RX processing oops

ieee80211_bss_info_update() can return NULL. Verify that this is not the
case before calling ieee802111_rx_bss_put() which would trigger an oops
in interrupt context in atomic_dec_and_lock().
Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/scan.c

@@ -388,6 +388,7 @@ ieee80211_scan_rx(struct ieee80211_sub_if_data *sdata, struct sk_buff *skb,
 	bss = ieee80211_bss_info_update(sdata->local, rx_status,
 					mgmt, skb->len, &elems,
 					freq, beacon);
+	if (bss)
 		ieee80211_rx_bss_put(sdata->local, bss);
 
 	dev_kfree_skb(skb);