chan_sip: Do not change IP address in SDP origin line (o=) in SIP reINVITE

If directmedia=yes is configured, when call is answered, Asterisk sends reINVITE
to both parties to set up media path directly between the endpoints.
In this reINVITE msg SDP origin line (o=) contains IP address of endpoint
instead of IP of asterisk. This behavior violates RFC3264, sec 8:
"When issuing an offer that modifies the session,
the "o=" line of the new SDP MUST be identical to that in the
previous SDP, except that the version in the origin field MUST
increment by one from the previous SDP."
This patch assures IP address of Asterisk is always sent in
SDP origin line.

ASTERISK-17540
Reported by:  saghul

Change-Id: I533a047490c43dcff32eeca8378b2ba02345b64e

channels/chan_sip.c

@@ -13512,12 +13512,13 @@ static enum sip_result add_sdp(struct sip_request *resp, struct sip_pvt *p, int
 
 	get_our_media_address(p, needvideo, needtext, &addr, &vaddr, &taddr, &dest, &vdest, &tdest);
 
+	/* We don't use dest here but p->ourip because address in o= field must not change in reINVITE */
 	snprintf(owner, sizeof(owner), "o=%s %d %d IN %s %s\r\n",
 		 ast_strlen_zero(global_sdpowner) ? "-" : global_sdpowner,
 		 p->sessionid, p->sessionversion,
-		 (ast_sockaddr_is_ipv6(&dest) && !ast_sockaddr_is_ipv4_mapped(&dest)) ?
+		 (ast_sockaddr_is_ipv6(&p->ourip) && !ast_sockaddr_is_ipv4_mapped(&p->ourip)) ?
 			"IP6" : "IP4",
-		 ast_sockaddr_stringify_addr_remote(&dest));
+		 ast_sockaddr_stringify_addr_remote(&p->ourip));
 
 	snprintf(connection, sizeof(connection), "c=IN %s %s\r\n",
 		 (ast_sockaddr_is_ipv6(&dest) && !ast_sockaddr_is_ipv4_mapped(&dest)) ?