compositor: Fix removal of an output

This uses the more safer version of signal emission to avoid a potential
crash when the output is destroyed that will follow a surface/view
destruction for which it has a listener attached (to the output_destroy
signal).

Fixes: #734

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/compositor.c

@@ -6414,8 +6414,8 @@ weston_compositor_remove_output(struct weston_output *output)
 	wl_list_insert(compositor->pending_output_list.prev, &output->link);
 	output->enabled = false;
 
-	wl_signal_emit(&compositor->output_destroyed_signal, output);
+	weston_signal_emit_mutable(&compositor->output_destroyed_signal, output);
-	wl_signal_emit(&output->destroy_signal, output);
+	weston_signal_emit_mutable(&output->destroy_signal, output);
 
 	wl_list_for_each(head, &output->head_list, output_link)
 		weston_head_remove_global(head);