txn_commit must abort txn before returning errors

Fix for failure when committing a txn that is a child of
some parent txn.

libraries/libmdb/mdb.c

@@ -1737,8 +1737,10 @@ mdb_txn_commit(MDB_txn *txn)
 		}
 		x = dst[0].mid;
 		for (; y<=src[0].mid; y++) {
-			if (++x >= MDB_IDL_UM_MAX)
+			if (++x >= MDB_IDL_UM_MAX) {
+				mdb_txn_abort(txn);
 				return ENOMEM;
+			}
 			dst[x] = src[y];
 		}
 		dst[0].mid = x;

libraries/libmdb/mdb.h

@@ -530,6 +530,7 @@ int  mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 *	<li>ENOSPC - no more disk space.
 	 *	<li>EIO - a low-level I/O error occurred while writing.
+	 *	<li>ENOMEM - the transaction is nested and could not be merged into its parent.
 	 * </ul>
 	 */
 int  mdb_txn_commit(MDB_txn *txn);