output/httpd: close client connection on error

This missing piece probably never really hurt, because
HttpdClient::OnSocketClosed() would be called right after a socket
error, but it's better to be explicit about closing on error.

src/output/plugins/httpd/HttpdClient.cxx

@@ -428,6 +428,7 @@ void
 HttpdClient::OnSocketError(std::exception_ptr ep) noexcept
 {
 	LogError(ep);
+	LockClose();
 }
 
 void