EVP: Fix the returned value for ASN1_PKEY_CTRL_DEFAULT_MD_NID

Trust the returned value from EVP_PKEY_get_default_digest_name()!  It
mimics exactly the values that EVP_PKEY_get_default_digest_nid() is
supposed to return, and that value should simply be passed unchanged.
Callers depend on it.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12586)

crypto/evp/p_lib.c

@@ -1202,19 +1202,18 @@ static int legacy_asn1_ctrl_to_param(EVP_PKEY *pkey, int op,
     case ASN1_PKEY_CTRL_DEFAULT_MD_NID:
         {
             char mdname[80] = "";
-            int nid;
             int rv = EVP_PKEY_get_default_digest_name(pkey, mdname,
                                                       sizeof(mdname));
 
-            if (rv <= 0)
-                return rv;
+            if (rv > 0) {
+                int nid;
+
                 nid = OBJ_sn2nid(mdname);
                 if (nid == NID_undef)
                     nid = OBJ_ln2nid(mdname);
-            if (nid == NID_undef)
-                return 0;
                 *(int *)arg2 = nid;
-            return 1;
+            }
+            return rv;
         }
     default:
         return -2;