chan_sip: Fix so asterisk won't send reINVITE after a BYE.

After a reINVITE glare situation, Asterisk would re-send the reINVITE
even though the call had been hung up in the mean time.  This patch
unschedules the reinvite when handling the BYE.

ASTERISK-22791 #close
Reported by: Paolo Compagnini
Tested by: Paolo Compagnini

Review: https://reviewboard.asterisk.org/r/4056/
(testcase is in review r4055)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@425296 f38db490-d61c-443f-a65b-d21fe96a405b