Fix crash introduced by 8e8a68c1b (Move code accessing bitmaps to
wxRibbonButtonBarButtonBase too, 2020-02-07) which start using
GetAncestorRibbonBar() in wxRibbonButtonBar code, without taking into
account that we may have no ribbon bar ancestor when the button bar is
shown in a popup frame, as it happens when there is not enough space to
show it fully in the main window itself.
Remember the ribbon bar in a member variable, relying on the fact that
we must have it in our parent chain when constructed, and just use it
later to avoid this.
Closes#19249.
706ab2c300