Prevent crashes on event for removed scroll bar on Haiku

Prevent crashes on event for removed scroll bar on Haiku

* src/haiku_support.cc (class EmacsScrollBar): Remove field
`scroll_bar'.
(ValueChanged, MouseDown, MouseUp, BScrollBar_make_for_view):
Adjust for changes.

* src/haiku_support.h (struct haiku_scroll_bar_value_event)
(struct haiku_scroll_bar_drag_event)
(struct haiku_scroll_bar_part_event): New field `window'.

* src/haikuterm.c (haiku_scroll_bar_from_widget): New function.
(haiku_read_socket): Adjust for changes in scroll bar event
structures.