PINGRESP from the network should not push the counter below zero

- finalize_pingresp() lowers m_pingTimeout every time, with no lower
limit. A broker can send a pile of unasked-for replies to push the
counter deep below zero, and then the client can spend some unexpected
amount of time to reach back the number that means:
"the broker has stopped answering, drop the link.”
- The fix prohibits pushing the counter below zero.

Task-number: QTBUG-149662
Pick-to: 6.11 6.12 6.8
Change-Id: I7b25e8ac3f83fa262a19010df9a91b2ee1c006fd
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>