staging: vt6656: check ieee80211_bss_conf bssid not NULL

Sometimes bssid can go null on failed association.
Signed-off-by: NMalcolm Priestley <tvboxspy@gmail.com>
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6656/main_usb.c

@@ -701,7 +701,7 @@ static void vnt_bss_info_changed(struct ieee80211_hw *hw,
 
 	priv->current_aid = conf->aid;
 
-	if (changed & BSS_CHANGED_BSSID)
+	if (changed & BSS_CHANGED_BSSID && conf->bssid)
 		vnt_mac_set_bssid_addr(priv, (u8 *)conf->bssid);