Not much information in a bug report: QByteArray is protected from negative
sizes and QUdpSocket too. FWIW - add one more check, similar to what
the server counterpart already had.
Pick-to: 5.15 6.0
Fixes: QTBUG-83457
Change-Id: I585fa90e0a258d2257e4fed2f24c52b47548bcbb
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>