rgw: fix for zonegroup redirect url

local dest_url variable was shadowing the one in the enclosing scope, so
the changes were not applied and no Location header was written on redirect

Fixes: http://tracker.ceph.com/issues/19488

Signed-off-by: Casey Bodley <cbodley@redhat.com>

src/rgw/rgw_rest.cc

@@ -775,7 +775,7 @@ void abort_early(struct req_state *s, RGWOp *op, int err_no,
       if (!s->redirect.empty()) {
         dest_uri = s->redirect;
       } else if (!s->zonegroup_endpoint.empty()) {
-        string dest_uri = s->zonegroup_endpoint;
+        dest_uri = s->zonegroup_endpoint;
         /*
          * reqest_uri is always start with slash, so we need to remove
          * the unnecessary slash at the end of dest_uri.