UPnP: better fix for postURL error that doesn't require behavioural b…

…reaks

src/network/upnp/UPnP.php

@@ -215,6 +215,7 @@ public static function portForward(string $serviceURL, string $internalIP, int $
 			'SOAPAction: "urn:schemas-upnp-org:service:WANIPConnection:1#AddPortMapping"'
 		];
 
+		$err = "";
 		if(Internet::postURL($serviceURL, $contents, 3, $headers, $err) === null){
 			throw new UPnPException("Failed to portforward using UPnP: " . $err);
 		}