boxes: avoid false positive warning in Clang static analyzer

avoid Clang static analyzer warning:

core/boxes.c:412:15: warning: Use of memory after it is freed
          if (meta_rectangle_contains_rect (a, b))
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 files changed, 3 insertions, 0 deletions

diff --git a/src/core/boxes.c b/src/core/boxes.c
index 19dd939e..7ae5f699 100644
--- a/src/core/boxes.c
+++ b/src/core/boxes.c
@@ -472,6 +472,9 @@ merge_spanning_rects_in_region (GList *region)
                   a = compare->data;
                 }
 
+              /* avoid false positive warning in Clang static analyzer */
+              g_assert (a != delete_me->data);
+
               /* Okay, we can free it now */
               g_free (delete_me->data);
               region = g_list_delete_link (region, delete_me);