madwifi: don't try to set an interface essid with an empty value

SVN-Revision: 15886

package/madwifi/files/lib/wifi/madwifi.sh

@@ -300,8 +300,10 @@ enable_atheros() {
 			config_set "$vif" bridge "$bridge"
 			start_net "$ifname" "$net_cfg"
 		}
-		[ -n "$ssid" ] && iwconfig "$ifname" essid on
-		iwconfig "$ifname" essid "$ssid"
+		[ -n "$ssid" ] && {
+			iwconfig "$ifname" essid on
+			iwconfig "$ifname" essid "$ssid"
+		}
 		set_wifi_up "$vif" "$ifname"
 
 		# TXPower settings only work if device is up already