Remove DSA digest length checks when no digest is passed

FIPS 186-4 does not specify a hard requirement on DSA digest lengths,
and in any case the current check rejects the FIPS recommended digest
lengths for key sizes != 1024 bits.

Fixes: #6748
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6749)

crypto/dsa/dsa_pmeth.c

@@ -77,13 +77,8 @@ static int pkey_dsa_sign(EVP_PKEY_CTX *ctx, unsigned char *sig,
     DSA_PKEY_CTX *dctx = ctx->data;
     DSA *dsa = ctx->pkey->pkey.dsa;
 
-    if (dctx->md) {
-        if (tbslen != (size_t)EVP_MD_size(dctx->md))
-            return 0;
-    } else {
-        if (tbslen != SHA_DIGEST_LENGTH)
-            return 0;
-    }
+    if (dctx->md != NULL && tbslen != (size_t)EVP_MD_size(dctx->md))
+        return 0;
 
     ret = DSA_sign(0, tbs, tbslen, sig, &sltmp, dsa);
 
@@ -101,13 +96,8 @@ static int pkey_dsa_verify(EVP_PKEY_CTX *ctx,
     DSA_PKEY_CTX *dctx = ctx->data;
     DSA *dsa = ctx->pkey->pkey.dsa;
 
-    if (dctx->md) {
-        if (tbslen != (size_t)EVP_MD_size(dctx->md))
-            return 0;
-    } else {
-        if (tbslen != SHA_DIGEST_LENGTH)
-            return 0;
-    }
+    if (dctx->md != NULL && tbslen != (size_t)EVP_MD_size(dctx->md))
+        return 0;
 
     ret = DSA_verify(0, tbs, tbslen, sig, siglen, dsa);