Handle a SIP race condition (reinvite before an ACK) properly.

Handle a SIP race condition (reinvite before an ACK) properly.

RFC 5047 explains the proper course of action to take if a
reINVITE is received before the ACK from a previous invite
transaction. What we are to do is to treat the reINVITE as
if it were both an ACK and a reINVITE and process it normally.

Later, when we receive the ACK we had been expecting, we will
ignore it since its CSeq is less than the current iseqno of
the sip_pvt representing this dialog.

(closes issue #13849)
Reported by: klaus3000
Patches:
      13849_v2.patch uploaded by mmichelson (license 60)
Tested by: mmichelson, klaus3000

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@187484 65c4cc65-6c06-0410-ace0-fbb531ad65f3