netutil: fix a buglet

I was noticing an excess number of conn objects remaining open after a
test shutdown.

Release note: None

pkg/util/netutil/net.go

@@ -191,6 +191,7 @@ func (s *TCPServer) ServeWith(
 			serveConn(ctx, rw)
 		})
 		if err != nil {
+			err = errors.CombineErrors(err, rw.Close())
 			return err
 		}
 	}