Added assert for lb_token before reffing it

src/core/ext/lb_policy/grpclb/grpclb.c

@@ -186,6 +186,7 @@ static void wrapped_rr_closure(grpc_exec_ctx *exec_ctx, void *arg,
      * addresses failed to connect). There won't be any user_data/token
      * available */
     if (wc_arg->target != NULL) {
+      GPR_ASSERT(wc_arg->lb_token != NULL);
       initial_metadata_add_lb_token(wc_arg->initial_metadata,
                                     wc_arg->lb_token_mdelem_storage,
                                     GRPC_MDELEM_REF(wc_arg->lb_token));