mac80211: fix potential use-after-free

The bss struct might be freed in ieee80211_rx_bss_put(),
so we shouldn't use it afterwards.

Cc: stable@vger.kernel.org (3.10+)
Fixes: 817cee76 ("mac80211: track AP's beacon rate and give it to the driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

@@ -2780,8 +2780,8 @@ static void ieee80211_rx_bss_info(struct ieee80211_sub_if_data *sdata,
 	bss = ieee80211_bss_info_update(local, rx_status, mgmt, len, elems,
 					channel);
 	if (bss) {
-		ieee80211_rx_bss_put(local, bss);
 		sdata->vif.bss_conf.beacon_rate = bss->beacon_rate;
+		ieee80211_rx_bss_put(local, bss);
 	}
 }