Merge 'btree: fix infinite looping in backwards iteration of btree table' from Jussi Saurio

Closes #1562
Existing "fuzz test" (not really fuzz, but kinda) didn't catch this due
to `LIMIT 3` clause

Closes #1563

testing/orderby.test

@@ -199,4 +199,15 @@ do_execsql_test orderby_desc_with_filter_id_le {
 6665
 6664
 6663
-6662}
+6662}
+
+# regression test where backwards iteration used to hang indefinitely
+do_execsql_test orderby_desc_regression {
+    select count(1) from (select * from users where id < 100 order by id desc)
+} {99}
+
+do_execsql_test orderby_desc_regression_verify_order {
+    select id from users where id < 100 order by id desc limit 3;
+} {99
+98
+97}