Lifetime of incoming QTcpSocket/QSslSocket are managed by the
QTcpServer/QSslServer and thus the QWebSocketPrivate should clear the
pointer to the socket when the socket is destroyed.
Added auto test for the crash.

Task-number: QTBUG-45331
Change-Id: I6e30e7cdcca22aa810ccc1a1d4d7d77c7cb2c349
Reviewed-by: Steven Ceuppens <steven.ceuppens@icloud.com>
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>