Simplify coding style of is_valid_ascii()
Calculate end of input rather than maintaining length,
per prior suggestion from Heikki Linnakangas. In passing,
use more natural language in a comment.
Discussion: https://www.postgresql.org/message-id/
b4648cc2-5e9c-c93a-52cc-
51e5c658a4f6%40iki.fi