QDBusConnectionPrivate: Use write lock inside handleSignal()

Slots like (un)registerServiceNoLock() modify shared state and
should be run with write lock taken. Having only read lock results
in hard to reproduce crashes.

Task-number: QTBUG-116621
Change-Id: I4456b64f9f2200378012822143e1a05345859d62
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusintegrator.cpp

@@ -1656,7 +1656,7 @@ void QDBusConnectionPrivate::handleSignal(const QDBusMessage& msg)
     key += u':';
     key += msg.interface();
 
-    QDBusReadLocker locker(HandleSignalAction, this);
+    QDBusWriteLocker locker(HandleSignalAction, this);
     handleSignal(key, msg);                  // one try
 
     key.truncate(msg.member().size() + 1); // keep the ':'