FileChooserDefault: Avoid printf()ing a NULL char*

If the name of the bookmark is NULL, using it with the printf %s format
specifier is undefined behaviour per the C Standard. Besides, it would
result in a completely unhelpful tooltip even if it were well-defined.

We already have an else case for when nothing is selected, which avoids
trying to use the name, so ensure we go there if it's NULL or empty too.

https://gitlab.gnome.org/GNOME/gtk/issues/1169

gtk/gtkfilechooserdefault.c

@@ -2810,8 +2810,8 @@ bookmarks_check_remove_sensitivity (GtkFileChooserDefault *impl)
 {
   GtkTreeIter iter;
   gboolean removable = FALSE;
+  gboolean have_name = FALSE;
   gchar *name = NULL;
-  gchar *tip;
   
   if (shortcuts_get_selected (impl, &iter))
     {
@@ -2821,6 +2821,13 @@ bookmarks_check_remove_sensitivity (GtkFileChooserDefault *impl)
                           -1);
       gtk_widget_set_sensitive (impl->browse_shortcuts_remove_button, removable);
 
+      have_name = name != NULL && name[0] != '\0';
+    }
+
+  if (have_name)
+    {
+      char *tip;
+
       if (removable)
         tip = g_strdup_printf (_("Remove the bookmark '%s'"), name);
       else