An QAbstractSocket can disconnect itself on flush(), which we call
after sending any packets. This leads to the packet protocol getting
deleted and in the next iteration of the loop
QmlDebugConnectionPrivate::readyRead() we'd use a null pointer.
Prevent that by checking for null.

Change-Id: Idfdb68a10c3e4deee7b305ff3b028247809090a6
Reviewed-by: hjk <hjk@theqtcompany.com>