Workaround VL32 cursor refcounting miscount

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

external/db_drivers/liblmdb/mdb.c

@@ -1953,13 +1953,15 @@ static void
 mdb_cursor_unref(MDB_cursor *mc)
 {
 	int i;
-	if (!mc->mc_snum || !mc->mc_pg[0] || IS_SUBP(mc->mc_pg[0]))
-		return;
-	for (i=0; i<mc->mc_snum; i++)
-		mdb_page_unref(mc->mc_txn, mc->mc_pg[i]);
-	if (mc->mc_ovpg) {
-		mdb_page_unref(mc->mc_txn, mc->mc_ovpg);
-		mc->mc_ovpg = 0;
+	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++)
+			mdb_page_unref(mc->mc_txn, mc->mc_pg[i]);
+		if (mc->mc_ovpg) {
+			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;