Fix a crash when accessing a dangling UploadDevice pointer #2984

Fix a crash when accessing a dangling UploadDevice pointer #2984

This is largely a guess, but this is the only place where we use
a QIODevice to push data through QNAM and that the QIODevice isn't
a direct child of the QNetworkReply.

Fix the issue by making sure that we don't go back to the event loop
and possibly handle network events between the destruction of the
upload QIODevice and the QNetworkReply, which might lead to QNAM
dereferencing a dangling QIODevice pointer.