entry: make sure priv->current_pos is valid

We can e.g. get the entry dispose()d and a focus_out event after that
(because the toplevel unsets the focus which previously was the entry).
We then later use priv->current_pos in a call to pango API which makes
sure the given index is valid for the given layout. Since we lazily
create a GtkEntryBuffer in get_buffer() and a PangoLayout lazily in
gtk_entry_create_layout, these 2 are always valid but don't match
priv->current_pos in this situation.

Fix this by resetting priv->current-pos in dispose().

https://bugzilla.gnome.org/show_bug.cgi?id=785255

gtk/gtkentry.c

@@ -2917,6 +2917,8 @@ gtk_entry_dispose (GObject *object)
   gtk_entry_set_icon_tooltip_markup (entry, GTK_ENTRY_ICON_SECONDARY, NULL);
   gtk_entry_set_completion (entry, NULL);
 
+  priv->current_pos = 0;
+
   if (priv->buffer)
     {
       buffer_disconnect_signals (entry);