compositor: do not overwrite error exit code

If there are several things that cause an exit, keep the error code from
the first one, not the last one. The latter ones could be just fallout
from the first.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor.c

@@ -4155,7 +4155,9 @@ WL_EXPORT void
 weston_compositor_exit_with_code(struct weston_compositor *compositor,
 				 int exit_code)
 {
+	if (compositor->exit_code == EXIT_SUCCESS)
 		compositor->exit_code = exit_code;
+
 	wl_display_terminate(compositor->wl_display);
 }