Cast the argument to unsigned char when calling isdigit()

Cast the argument to unsigned char when calling isdigit()

Fixes #21123

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21127)