Fix possible crash with nested SubLinks.

An expression such as WHERE (... x IN (SELECT ...) ...) IN (SELECT ...)
could produce an invalid plan that results in a crash at execution time,
if the planner attempts to flatten the outer IN into a semi-join.
This happens because convert_testexpr() was not expecting any nested
SubLinks and would wrongly replace any PARAM_SUBLINK Params belonging
to the inner SubLink.  (I think the comment denying that this case could
happen was wrong when written; it's certainly been wrong for quite a long
time, since very early versions of the semijoin flattening logic.)

Per report from Teodor Sigaev.  Back-patch to all supported branches.

src/test/regress/expected/subselect.out

@@ -710,3 +710,32 @@ select exists(select * from nocolumns);
  f
 (1 row)
 
+--
+-- Check sane behavior with nested IN SubLinks
+--
+explain (verbose, costs off)
+select * from int4_tbl where
+  (case when f1 in (select unique1 from tenk1 a) then f1 else null end) in
+  (select ten from tenk1 b);
+                                                                                      QUERY PLAN                                                                                       
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Nested Loop Semi Join
+   Output: int4_tbl.f1
+   Join Filter: (CASE WHEN (hashed SubPlan 1) THEN int4_tbl.f1 ELSE NULL::integer END = b.ten)
+   ->  Seq Scan on public.int4_tbl
+         Output: int4_tbl.f1
+   ->  Seq Scan on public.tenk1 b
+         Output: b.unique1, b.unique2, b.two, b.four, b.ten, b.twenty, b.hundred, b.thousand, b.twothousand, b.fivethous, b.tenthous, b.odd, b.even, b.stringu1, b.stringu2, b.string4
+   SubPlan 1
+     ->  Index Only Scan using tenk1_unique1 on public.tenk1 a
+           Output: a.unique1
+(10 rows)
+
+select * from int4_tbl where
+  (case when f1 in (select unique1 from tenk1 a) then f1 else null end) in
+  (select ten from tenk1 b);
+ f1 
+----
+  0
+(1 row)
+

src/test/regress/sql/subselect.sql

@@ -411,3 +411,14 @@ explain (verbose, costs off)
 --
 create temp table nocolumns();
 select exists(select * from nocolumns);
+
+--
+-- Check sane behavior with nested IN SubLinks
+--
+explain (verbose, costs off)
+select * from int4_tbl where
+  (case when f1 in (select unique1 from tenk1 a) then f1 else null end) in
+  (select ten from tenk1 b);
+select * from int4_tbl where
+  (case when f1 in (select unique1 from tenk1 a) then f1 else null end) in
+  (select ten from tenk1 b);