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)
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Merged revisions 423720 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 423721 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 423722 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423723 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423724 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -25771,7 +25771,7 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, str
 			break;
 		}
 	} else {
-		if (p && (p->autokillid == -1)) {
+		if (!req->ignore && p && (p->autokillid == -1)) {
 			const char *msg;
 			if ((!ast_format_cap_count(p->jointcaps)))