[gtk/builder-leaks] Add an assertion and comment

[Matthias Clasen <matthiasc src gnome org>]

commit e3ecd5d23e6078b01e86c6002fd9de75a125f910
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Sep 27 17:17:06 2021 -0400

    Add an assertion and comment
    
    Maybe it helps the person unfortunate enough
    to stumble in here.

 gtk/gtkbuilder.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/gtk/gtkbuilder.c b/gtk/gtkbuilder.c
index 6f13bde4c7..a151ec78cf 100644
--- a/gtk/gtkbuilder.c
+++ b/gtk/gtkbuilder.c
@@ -606,6 +606,11 @@ gtk_builder_get_parameters (GtkBuilder         *builder,
           continue;
         }
 
+      /* At this point, property_value has been set, and we need to either
+       * copy it to one of the two arrays, or unset it.
+       */
+      g_assert (G_IS_VALUE (&property_value));
+
       if ((prop->pspec->flags & filter_flags) != 0 && filtered_parameters)
         object_properties_add (filtered_parameters, property_name, &property_value);
       else if ((prop->pspec->flags & filter_flags) == 0 && parameters)