mdb_dcmp(): Assume the database has MDB_DUPSORT.

There was little point in returning EINVAL when not: Comparing (A,B)
and (B,A) would claim (A > B && B > A), which could confuse callers.

libraries/liblmdb/lmdb.h

@@ -1252,7 +1252,7 @@ int  mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
 	/** @brief Compare two data items according to a particular database.
 	 *
 	 * This returns a comparison as if the two items were data items of
-	 * a sorted duplicates #MDB_DUPSORT database.
+	 * the specified database. The database must have the #MDB_DUPSORT flag.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] a The first item to compare

libraries/liblmdb/mdb.c

@@ -1246,10 +1246,7 @@ mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b)
 int
 mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b)
 {
-	if (txn->mt_dbxs[dbi].md_dcmp)
-		return txn->mt_dbxs[dbi].md_dcmp(a, b);
-	else
-		return EINVAL;	/* too bad you can't distinguish this from a valid result */
+	return txn->mt_dbxs[dbi].md_dcmp(a, b);
 }
 
 /** Allocate a single page.