Fix infinite DTMF when a BEGIN is received without an END.

Fix infinite DTMF when a BEGIN is received without an END.

This commit is related to rev 175124 of 1.4 where a previous attempt was made
to fix this problem.  The problem with the previous patch was that the inserted
code needed to go _before_ setting the lastrxts to the current timestamp.
Because those were the same, the dtmfcount variable was never decremented, and
so the END was never sent.

In passing, I removed the dtmfsamples variable which was completed unused.  I
also removed a redundant setting of the lastrxts variable.

(closes issue #14460)
Reported by: moliveras

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@178141 65c4cc65-6c06-0410-ace0-fbb531ad65f3