[gtk/prop-list] builder: Avoid a crash in error cases

[Matthias Clasen <matthiasc src gnome org>]

commit 6d30913f0200fdf8591f09082c3769b9ab98057b
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Dec 23 17:58:15 2019 -0500

    builder: Avoid a crash in error cases
    
    Don't leave a dangling pointer behind. I've hit
    cases where we crash trying to free it again.

 gtk/gtkbuilderparser.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/gtkbuilderparser.c b/gtk/gtkbuilderparser.c
index f7a323b508..5cfb8c8119 100644
--- a/gtk/gtkbuilderparser.c
+++ b/gtk/gtkbuilderparser.c
@@ -1379,6 +1379,7 @@ expression_info_construct (GtkBuilder      *builder,
             if (expression == NULL)
               return NULL;
             free_expression_info (info->property.expression);
+            info->property.expression = NULL;
           }
         else
           expression = NULL;