[glade] GladeUtil: add guard to glade_util_get_file_mtime()

[Juan Pablo Ugarte <jpu src gnome org>]

commit b4f12e73613700e344ac32a4c64cd8a067a59d1a
Author: Juan Pablo Ugarte <juanpablougarte gmail com>
Date:   Fri Oct 9 17:42:30 2020 -0300

    GladeUtil: add guard to glade_util_get_file_mtime()
    
    Closes issue #98

 gladeui/glade-utils.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gladeui/glade-utils.c b/gladeui/glade-utils.c
index e8cfecbc..b23e297a 100644
--- a/gladeui/glade-utils.c
+++ b/gladeui/glade-utils.c
@@ -1280,6 +1280,8 @@ glade_util_get_file_mtime (const gchar *filename, GError **error)
   struct stat info;
   gint retval;
 
+  g_return_val_if_fail (filename, 0);
+
   retval = g_stat (filename, &info);
 
   if (retval != 0)