staging: vt6655: vnt_bss_info_changed check conf->beacon_rate is not NULL

conf->beacon_rate can be NULL on association. So check conf->beacon_rate

BSS_CHANGED_BEACON_INFO needs to flagged in changed as the beacon_rate
will appear later.
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Cc: <stable@vger.kernel.org> # v3.19+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6655/device_main.c

@@ -1483,8 +1483,9 @@ static void vnt_bss_info_changed(struct ieee80211_hw *hw,
 		}
 	}
 
-	if (changed & BSS_CHANGED_ASSOC && priv->op_mode != NL80211_IFTYPE_AP) {
-		if (conf->assoc) {
+	if (changed & (BSS_CHANGED_ASSOC | BSS_CHANGED_BEACON_INFO) &&
+	    priv->op_mode != NL80211_IFTYPE_AP) {
+		if (conf->assoc && conf->beacon_rate) {
 			CARDbUpdateTSF(priv, conf->beacon_rate->hw_value,
 				       conf->sync_tsf);