Torrent example: don't try to announce to an empty URL

Task-number: QTBUG-109798
Task-number: QTBUG-110622
Pick-to: 6.5
Change-Id: I3603232eb094387388da6421bf5525a3ba54fe03
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

examples/network/torrent/trackerclient.cpp

@@ -54,6 +54,8 @@ void TrackerClient::timerEvent(QTimerEvent *event)
 
 void TrackerClient::fetchPeerList()
 {
+    if (metaInfo.announceUrl().isEmpty())
+        return;
     QUrl url(metaInfo.announceUrl());
 
     // Base the query on announce url to include a passkey (if any)