Workaround VL32 cursor refcounting miscount

Don't try to deref cursor page if txn's pagelist is empty

libraries/liblmdb/mdb.c

@@ -2006,6 +2006,7 @@ static void
 mdb_cursor_unref(MDB_cursor *mc)
 {
 	int i;
+	if (mc->mc_txn->mt_rpages[0].mid) {
 		if (!mc->mc_snum || !mc->mc_pg[0] || IS_SUBP(mc->mc_pg[0]))
 			return;
 		for (i=0; i<mc->mc_snum; i++)
@@ -2014,6 +2015,7 @@ mdb_cursor_unref(MDB_cursor *mc)
 			mdb_page_unref(mc->mc_txn, mc->mc_ovpg);
 			mc->mc_ovpg = 0;
 		}
+	}
 	mc->mc_snum = mc->mc_top = 0;
 	mc->mc_pg[0] = NULL;
 	mc->mc_flags &= ~C_INITIALIZED;