chan_sip: Fix dialog leak resulting from missing ACK to re-INVITE.

If a device re-INVITEs at the same time as the dialog is hung up, and
if then the ACK to the re-INVITE never reaches Asterisk, chan_sip would
fail to destroy the dialog after a while.  This resulted in (most
prominently) file handle leaks.

(Patch reindented by me.)

ASTERISK-20784 #close
ASTERISK-15879 #close
Reported by: Torrey Searle, Nitesh Bansal
Patches:
  reinvite_ack_timeout.patch uploaded by Torrey Searle (License #5334)
  patch_asterisk_20784.txt uploaded by Nitesh Bansal (License #6418)

Reviewboard: https://reviewboard.asterisk.org/r/4052/
(testcase can be found at r4051)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@425068 f38db490-d61c-443f-a65b-d21fe96a405b