Check whether connection is still active upon mio event

Fix issue #3

src/tun2proxy.rs

@@ -439,8 +439,8 @@ impl<'a> TunToProxy<'a> {
     }
 
     fn mio_socket_event(&mut self, event: &Event) {
-        let connection = *self.token_to_connection.get(&event.token()).unwrap();
+        if let Some(conn_ref) = self.token_to_connection.get(&event.token()) {
-
+            let connection = *conn_ref;
             if event.is_readable() {
                 {
                     let state = self.connections.get_mut(&connection).unwrap();
@@ -496,6 +496,7 @@ impl<'a> TunToProxy<'a> {
                 self.write_to_server(&connection);
             }
         }
+    }
 
     fn udp_event(&mut self, _event: &Event) {}