QTuioHander: Use the returned size to ensure that the QByteArray is correct.

If the initial resize() was too large compared to the actual read, this will
correct it to ensure we don't try read garbage.
QTuioHandler: Check for readDatagram failure, and skip parsing if it happens.
Add a comment pointing to the spec behaviour.
Remove an unnecessary comment.

This is safe.
Add an API note that requires fixing.