QOrderedMutexLocker: use std::less to prevent undefined behavior

operator< between pointers is undefined unless the two pointers
point in the same array, which is not what QOrderedMutexLocker does.

Change-Id: Ia6594900cfa807a73f20e157ce896b4321a3d746
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/thread/qorderedmutexlocker_p.h

@@ -47,6 +47,8 @@
 
 #include <QtCore/qmutex.h>
 
+#include <functional>
+
 QT_BEGIN_NAMESPACE
 
 /*
@@ -57,8 +59,8 @@ class QOrderedMutexLocker
 {
 public:
     QOrderedMutexLocker(QMutex *m1, QMutex *m2)
-        : mtx1((m1 == m2) ? m1 : (m1 < m2 ? m1 : m2)),
-          mtx2((m1 == m2) ?  0 : (m1 < m2 ? m2 : m1)),
+        : mtx1((m1 == m2) ? m1 : (std::less<QMutex *>()(m1, m2) ? m1 : m2)),
+          mtx2((m1 == m2) ?  0 : (std::less<QMutex *>()(m1, m2) ? m2 : m1)),
           locked(false)
     {
         relock();