Fix crash when calling QObject::disconnect for QNetworkReplyWrapper

Fix it by watching the QNetworkReply's destroyed() signal and avoid the dangling pointer
instead. The QNetworkReply doesn't need to be aborted in this case anyway.

Issue: https://github.com/ariya/phantomjs/issues/11252
Upstream bug: https://bugs.webkit.org/show_bug.cgi?id=116035