fix potential crash

src/core/ext/filters/client_channel/lb_policy/xds/xds.cc

@@ -1099,7 +1099,9 @@ void XdsLb::LocalityMap::UpdateXdsPickerLocked() {
     const auto& locality_name = p.first;
     Locality* locality = p.second.get();
     // Skip the localities that are not in the latest locality map update.
-    if (!locality_map_update()->Contains(locality_name)) continue;
+    const auto* locality_update = locality_map_update();
+    if (locality_update == nullptr) continue;
+    if (!locality_update->Contains(locality_name)) continue;
     if (locality->connectivity_state() != GRPC_CHANNEL_READY) continue;
     end += locality->weight();
     picker_list.push_back(