libweston: Assert if ref-count balance is wrong

Calling weston_surface_destroy() one time too many could be a sign we
haven't correctly increased the ref count for it.

Also, if we don't have a surface being passed, do no attempt to
use it.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
(cherry picked from commit d3ed2eb345)

libweston/compositor.c

@@ -2306,6 +2306,10 @@ weston_surface_destroy(struct weston_surface *surface)
 	struct weston_pointer_constraint *constraint, *next_constraint;
 	struct weston_paint_node *pnode, *pntmp;
 
+	if (!surface)
+		return;
+
+	assert(surface->ref_count > 0);
 	if (--surface->ref_count > 0)
 		return;