Fix strict-warnings build

crypto/asn1/a_strex.c: Type of width variable in asn1_valid_host
function  needs to be changed from char to signed char to avoid
build error due to '-Werror=type-limits'.

Signed-off-by: Patrick Steuer <psteuer@mail.de>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
CLA: trivial