[gtk/builder-tool-placeholder-crash] gtk-builder-tool: Avoid a crash with <placeholder/>

[Matthias Clasen <matthiasc src gnome org>]

commit 6a66ccd3e6b8360d4c3ada1456c8423eebdea065
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Mar 23 15:24:30 2021 -0400

    gtk-builder-tool: Avoid a crash with <placeholder/>
    
    Not sure we can do much with a ui file that has
    <placeholder/> elements littered throughout. But
    at least we can avoid crashing while trying to
    convert it.
    
    Fixes: #3781

 tools/gtk-builder-tool-simplify.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/tools/gtk-builder-tool-simplify.c b/tools/gtk-builder-tool-simplify.c
index 990377bd21..22e41a34d7 100644
--- a/tools/gtk-builder-tool-simplify.c
+++ b/tools/gtk-builder-tool-simplify.c
@@ -944,6 +944,8 @@ rewrite_notebook_page (Element *child, Element *tab, MyParserData *data)
         object = elt;
       else if (g_str_equal (elt->element_name, "packing"))
         packing = elt;
+      else if (g_str_equal (elt->element_name, "placeholder"))
+        return child;
     }
 
   if (!packing && !tab)