tolower: refine the tolower code to avoid a memory access

tolower: refine the tolower code to avoid a memory access

This improves the performance of this function and the ones that rely on it
(ossl_lh_strcasehash primarily).

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/18344)