qmljs: improve multithreading safety

ensure that the QSet copy operation of the scannedPaths in importScan
is atomic.

Change-Id: Ifb3566a86d1d6ac6abf29f1b4cbe19868947d0cc
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/libs/qmljs/qmljsmodelmanagerinterface.cpp

@@ -947,7 +947,11 @@ void ModelManagerInterface::importScan(QFutureInterface<void> &future,
                               bool emitDocChangedOnDisk)
 {
     // paths we have scanned for files and added to the files list
-    QSet<QString> scannedPaths = modelManager->m_scannedPaths;
+    QSet<QString> scannedPaths;
+    {
+        QMutexLocker l(&modelManager->m_mutex);
+        scannedPaths = modelManager->m_scannedPaths;
+    }
     // libraries we've found while scanning imports
     QSet<QString> newLibraries;