chan_sip: On INVITE retransmission, don't add an extra 503 response.

chan_sip: On INVITE retransmission, don't add an extra 503 response.

INVITE arrives to asterisk, asterisk responds Busy(). If the INVITE is
retransmitted, asterisk would generate a 503 in addition to the 486.

Thanks Torrey Searle for providing a working regression test.

ASTERISK-24335 #close

Review: https://reviewboard.asterisk.org/r/4003/
Patches:
  retrans_486_invite.patch uploaded by Torrey Searle (License #5334)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@423720 65c4cc65-6c06-0410-ace0-fbb531ad65f3