Replace unsigned with int

Replace unsigned with int

Replace the type of "digest_length" with int to avoid implicit conversion when it is assigned by EVP_MD_get_size().
Otherwise, it may pass the following check and cause the integer overflow error when EVP_MD_get_size() returns negative numbers.
Signed-off-by: Jiasheng Jiang <jiasheng@purdue.edu>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23922)