Don't do anything if setting the same proxy factory.

Otherwise, it crashes if you simply set the same proxy factory instance
twice.

Change-Id: I4b1f1d0eed7630c9a145c0a330c6be1b3fc381e5
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>

src/network/kernel/qnetworkproxy.cpp

@@ -278,6 +278,8 @@ public:
     void setApplicationProxyFactory(QNetworkProxyFactory *factory)
     {
         QMutexLocker lock(&mutex);
+        if (factory == applicationLevelProxyFactory)
+            return;
         if (applicationLevelProxy)
             *applicationLevelProxy = QNetworkProxy();
         delete applicationLevelProxyFactory;