Fix region extension when scroll bar is on the right

Fix region extension when scroll bar is on the right

 src/keyboard.c (make_lispy_position): When the click is on the
 right-side vertical scroll bar, pass the rightmost X coordinate to
 buffer_posn_from_coords, so that the returned text position
 reflects the closest point to the click.  Fixes region extension
 when mouse moves outside the Emacs frame that has scroll bars on
 the right.