lib: Clear state in an error path

Ensure that the UhmServer’s internal state is unchanged if
uhm_server_start_trace_full() hits an error.

libuhttpmock/uhm-server.c

@@ -1636,6 +1636,7 @@ uhm_server_start_trace_full (UhmServer *self, GFile *trace_file, GError **error)
 			g_error_free (child_error);
 
 			uhm_server_stop (self);
+			g_clear_object (&priv->output_stream);
 
 			return;
 		}
@@ -1650,6 +1651,8 @@ uhm_server_start_trace_full (UhmServer *self, GFile *trace_file, GError **error)
 
 			g_error_free (child_error);
 
+			g_clear_object (&priv->output_stream);
+
 			return;
 		}
 	}