Fix reentrancy problem in image reading

There were race conditions when accessing the plugin factory and
the image reader plugins from different threads; ref QTBUG-29281.
Added a mutex lock to avoid.

Change-Id: Ic1a3b6cbaf5603f1bcf7025b58247a9a3f6d08a9
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/gui/image/qimagereader.cpp

@@ -133,6 +133,7 @@
 // factory loader
 #include <qcoreapplication.h>
 #include <private/qfactoryloader_p.h>
+#include <QMutexLocker>
 
 // image handlers
 #include <private/qbmphandler_p.h>
@@ -232,6 +233,9 @@ static QImageIOHandler *createReadHandlerHelper(QIODevice *device,
     QByteArray suffix;
 
 #ifndef QT_NO_IMAGEFORMATPLUGIN
+    static QMutex mutex;
+    QMutexLocker locker(&mutex);
+
     typedef QMultiMap<int, QString> PluginKeyMap;
 
     // check if we have plugins that support the image format