store/loader_file.c: fix char-subscripts warning.

This happens on systems that perform is* character classifictions as
array lookup, e.g. NetBSD.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6584)