Fix coverity-1604666

Fix coverity-1604666

Coverity recently flaged an error in which the return value for
EVP_MD_get_size wasn't checked for negative values prior to use, which
can cause underflow later in the function.

Just add the check and error out if get_size returns an error.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24896)