mac80211: accept empty strings for hidden SSIDs

Some access points (e.g. Sitecom WL-174) use an empty string as hidden SSID.
Signed-off-by: Fabio Rossi <rossi.f@inwind.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/mlme.c

@@ -2012,7 +2012,7 @@ static int ieee80211_sta_match_ssid(struct ieee80211_if_sta *ifsta,
 		}
 	}
 
-	if (hidden_ssid && ifsta->ssid_len == ssid_len)
+	if (hidden_ssid && (ifsta->ssid_len == ssid_len || ssid_len == 0))
 		return 1;
 
 	if (ssid_len == 1 && ssid[0] == ' ')