beam.cc: avoid calculating an invalid reference

beam.cc: avoid calculating an invalid reference

In calc_beam_segments, an invalid reference to a neighboring segment
was calculated (and not used) at the extreme ends of the beam.  This
caused segfaults on Fedora.  Since the reference is used at most twice
anyway and in the same expression, it seems easier to just use the
expression directly (the invalid cases are then skipped due to
short-circuit evaluation).

[mtasaka, 2018-05-09 15:07:04]

I checked the patch and it looks good. Also I've confirmed that applying this
patch actually prevents abort for reported rest-pitched-beam.ly and
as-the-deer.ly (as-the-deer.ly was on RedHat bugzilla).

[dak, 2018-05-09 15:20:32]

On 2018/05/09 15:07:04, mtasaka wrote:
> I checked the patch and it looks good. Also I've confirmed that applying this
> patch actually prevents abort for reported rest-pitched-beam.ly and
> as-the-deer.ly (as-the-deer.ly was on RedHat bugzilla).

Thanks, I'll take that as an LGTM then.