entry: make sure priv->current_pos is valid

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().