Fix crash in link to destroyed agents (#1827)

The destroyed field on the agent state was never set, so the early
return in `run` didn't prevent crashes when agents receive messages
after they are destroyed.

packages/yew/src/agent/link.rs

@@ -207,6 +207,7 @@ where
                     .take()
                     .expect("trying to destroy not existent agent");
                 agent.destroy();
+                state.destroyed = true;
             }
         }
     }