Fix crash in qmake parsing

The read from a QHash needs to be protected too if other threads are
writing.

sync-up with qtc, no actual effect on qmake itself.

Fixes: QTCREATORBUG-21416
Change-Id: I75e5634e11b10056d6dbb6fdceef482ca2222ca1
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
(cherry picked from qtcreator/5f79b5d2e5e33321cdcd00362f0d6d9442a73ec2)
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

qmake/library/qmakevfs.cpp

@@ -109,10 +109,10 @@ int QMakeVfs::idForFileName(const QString &fn, VfsFlags flags)
             return id;
     }
 #endif
-    if (!(flags & VfsAccessedOnly)) {
 #ifdef PROPARSER_THREAD_SAFE
-        QMutexLocker locker(&s_mutex);
+    QMutexLocker locker(&s_mutex);
 #endif
+    if (!(flags & VfsAccessedOnly)) {
         int &id = s_fileIdMap[fn];
         if (!id) {
             id = ++s_fileIdCounter;