Re: Small improvements to substring()

[Thomas Munro]

Thanks for looking, Chao and Junwang.  It's a straightforward win, but
I'll wait a week or two for the dust to settle on the recent bugfix
and for any other reviews or objections.  That'll give master a bit of
extra time with code matching the release branches, for the unlikely
case that developers working on master discover something interesting
about that.