Due to the way stringfields work the value of the url pointer will always be non-NULL so we have to use ast_strlen_zero to make sure it is not empty. (issue #9821 reported by pj)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@66314 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -6150,7 +6150,7 @@ static int respprep(struct sip_request *resp, struct sip_pvt *p, const char *msg
 		add_header(resp, "Contact", p->our_contact);
 	}
 
-	if (p->url) {
+	if (!ast_strlen_zero(p->url)) {
 		add_header(resp, "Access-URL", p->url);
 		ast_string_field_free(p, url);
 	}
@@ -6260,7 +6260,7 @@ static int reqprep(struct sip_request *req, struct sip_pvt *p, int sipmethod, in
 	if (!ast_strlen_zero(p->rpid))
 		add_header(req, "Remote-Party-ID", p->rpid);
 
-	if (p->url) {
+	if (!ast_strlen_zero(p->url)) {
 		add_header(req, "Access-URL", p->url);
 		ast_string_field_free(p, url);
 	}