Set sip_request ast_str data to NULL so ast_str_copy allocates space properly

in copy_request

(issue #14478)
Reported by: erik_dedecker



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@177624 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -19497,6 +19497,7 @@ static int handle_request_refer(struct sip_pvt *p, struct sip_request *req, int
 					/* Chan2: Call between asterisk and transferee */
 	int res = 0;
+	current.req.data = NULL;
 	if (req->debug)
 		ast_verbose("Call %s got a SIP call transfer from %s: (REFER)!\n", p->callid, ast_test_flag(&p->flags[0], SIP_OUTGOING) ? "callee" : "caller");