Do not send a BYE when a provisional response arrives during a re-INVITE

Do not send a BYE when a provisional response arrives during a re-INVITE

Commits r369557 and r369579 were done to improve handling of re-INVITEs
when the UA that was supposed to receive the re-INVITE fails to respond.
A limitation of those patches occurred when a UA sent a provisional
response to the re-INVITE.  This triggered a sending of a BYE in
check_pending.  This patch tweaks the handling of the re-INVITE such that
a BYE is not sent in response to those messages.

(issue ASTERISK-19992)
Reported by: Steve Davies
Tested by: Steve Davies
patches:
  (reinvite_tweak.diff license #5012 by Steve Davies)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@369626 65c4cc65-6c06-0410-ace0-fbb531ad65f3