Add nullptr check in QTcpServer::nextPendingConnection

The following line is causing a crash in QTcpServer::nextPendingConnection if QTcpServer instance doesn't listen (socketEngine is null).

if (!d->socketEngine->isReadNotificationEnabled()) 

With the nullptr check for d->socketEngine it wouldn't cause this crash anymore.

if (d->socketEngine && !d->socketEngine->isReadNotificationEnabled())