pgsql: Simplify coding style of is_valid_ascii()

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6b41a1579bd37aa95924d592d7a18a79d669662f

Modified Files
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src/include/mb/pg_wchar.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)