BUG/MINOR: quic: Possible wrapped values used as ACK tree purging limit.

BUG/MINOR: quic: Possible wrapped values used as ACK tree purging limit.

Add two missing checks not to substract too big values from another too little
one. In this case the resulted wrapped huge values could be passed to the function
which has to remove the last range of a tree of ACK ranges as encoded limit size
not to go below, cancelling the ACK ranges deletion. The consequence could be that
no ACK were sent.

Must be backported to 2.6 and 2.7.