Debugger: Only use random QML port if none was given

This is a cherry-pick of 20db17e4f0 from
4.1. The problem also occurs in 4.0 and as another version of 4.0 might
be released, we should fix it there, too.

Change-Id: I3cf42abfa9d0ea5812495be6e2728264dc115ba9
Task-number: QTCREATORBUG-16708
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/debuggerruncontrol.cpp

@@ -450,15 +450,18 @@ void DebuggerRunControlCreator::enrich(const RunConfiguration *runConfig, const
 
     if (m_rp.languages & QmlLanguage) {
         if (m_rp.device && m_rp.device->type() == ProjectExplorer::Constants::DESKTOP_DEVICE_TYPE) {
+            if (m_rp.qmlServerAddress.isEmpty() || m_rp.qmlServerPort == InvalidPort) {
                 QTcpServer server;
                 const bool canListen = server.listen(QHostAddress::LocalHost)
                         || server.listen(QHostAddress::LocalHostIPv6);
                 if (!canListen) {
-                m_errors.append(DebuggerPlugin::tr("Not enough free ports for QML debugging.") + QLatin1Char(' '));
+                    m_errors.append(DebuggerPlugin::tr("Not enough free ports for QML debugging.")
+                                    + QLatin1Char(' '));
                     return;
                 }
                 m_rp.qmlServerAddress = server.serverAddress().toString();
                 m_rp.qmlServerPort = server.serverPort();
+            }
 
             // Makes sure that all bindings go through the JavaScript engine, so that
             // breakpoints are actually hit!