Issue 1553: Staff stretching problem in 2.13.50+

optimal-page-breaking.cc needs to protect unsigned subtraction.

Signed-off-by: David Kastrup <dak@gnu.org>
Release: bump VERSION.
Release: update news.
 Issue 3676: Bar checks display the wrong location

This one is just embarrassing.  The lexer used last_input_ where it
should have been using here_input (), resulting in music identifiers
and some other stuff to be located one token early.