Fix send on closed channel

If a matcher go-routine mateches a connection after the root
listener was closed, cmux will panic since all connection channels
will be closed. This commit avoids closing connection channels but
signals the matcher go-routines that the listener is closed via
a done channel.

Tested: Added a test case for this issue.

cmux_test.go

@@ -168,3 +168,16 @@ func TestErrorHandler(t *testing.T) {
 		t.Error("rpc got a response")
 	}
 }
+
+type closerConn struct {
+	net.Conn
+}
+
+func (c closerConn) Close() error { return nil }
+
+func TestClosed(t *testing.T) {
+	mux := &cMux{}
+	lis := mux.Match(Any()).(muxListener)
+	close(lis.donec)
+	mux.serve(closerConn{})
+}