fix(mtcp): re-init session in Handshake when it has no mux session yet

A session Dialed but not yet Handshaken has session.session == nil; a
concurrent Dial treated it as dead (IsClosed() is true for nil sessions),
deleted it and closed the base conn under the first caller's Handshake.
Re-initializing on the nil-session case self-heals instead of failing
the identity check.

dialer/mtcp/dialer.go

@@ -119,7 +119,7 @@ func (d *mtcpDialer) Handshake(ctx context.Context, conn net.Conn, options ...di
 	session, ok := d.sessions[opts.Addr]
 	if session != nil && session.conn != conn {
 		conn.Close()
-		return nil, errors.New("mtls: unrecognized connection")
+		return nil, errors.New("mtcp: unrecognized connection")
 	}
 
 	if !ok || session.session == nil {