* Add a lock to be used in the update_call_counter function.

* Revert logic to mirror 1.4's in the sense that it will not allow
  the call counter to dip below 0.

These two measures prevent potential races that could cause a SIP peer
to appear to be busy forever.

(closes issue #13668)
Reported by: mjc
Patches:
      hintfix_trunk_rev152649.patch uploaded by wolfelectronic (license 586)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@157427 65c4cc65-6c06-0410-ace0-fbb531ad65f3