Minor QObject::receivers() optimization.

Don't lock if there are no receivers.

Change-Id: I922fad10436adeb27aa4f193cc0dcbaa3f0c0f1e
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qobject.cpp

@@ -2190,6 +2190,9 @@ int QObject::receivers(const char *signal) const
                                                              metaObject()->indexOfMethod(signal));
         }
 
+        if (!d->isSignalConnected(signal_index))
+            return receivers;
+
         QMutexLocker locker(signalSlotLock(this));
         if (d->connectionLists) {
             if (signal_index < d->connectionLists->count()) {