Make sure to reenable echo can after a "failed" (canceled, etc) three-way call.

(closes issue #11335)
Reported by: rebuild


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@107173 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_zap.c

@@ -4571,6 +4571,7 @@ static struct ast_frame *__zt_exception(struct ast_channel *ast)
 			update_conf(p);
 			break;
 		case ZT_EVENT_RINGOFFHOOK:
+			zt_enable_ec(p);
 			zt_set_hook(p->subs[SUB_REAL].zfd, ZT_OFFHOOK);
 			if (p->owner && (p->owner->_state == AST_STATE_RINGING)) {
 				p->subs[SUB_REAL].needanswer = 1;