Null pointer checking missing in QNetworkSession

Null pointer checking for d in QNetworkSession in Mobility is missing, causing occasional crashes.

One crash was found during overnight testing of a third party application (log attached)

In this particular case, the crash seems to have happend in the following function.

void QNetworkSession::close()
{
    d->close();
}

In QNetworkSession in QtNetwork, d is always checked for validity before it is used. Same thing should be done in Mobility.