don't check 'acknowledged' for setting lastdisc, since it's guaranteed to be...

don't check 'acknowledged' for setting lastdisc, since it's guaranteed to be zero at this point (issue #5254)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@6736 65c4cc65-6c06-0410-ace0-fbb531ad65f3