signature/dsa_sig.c: Add checks for the EVP_MD_get_size()

signature/dsa_sig.c: Add checks for the EVP_MD_get_size()

Add checks for the EVP_MD_get_size() to avoid integer overflow and then explicitly cast from int to size_t.

Fixes: 45a845e40b ("Add EVP_DigestSign/EVP_DigestVerify support for DSA")
Signed-off-by: Jiasheng Jiang <jiasheng@purdue.edu>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23948)