pgsql: Fix possible crash with nested SubLinks.

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.

Branch
------
REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/884c6384a2db34f6a65573e6bfd4b71dfba0de90

Modified Files
--------------
src/backend/optimizer/plan/subselect.c  |   27 ++++++++++++++++++++++-----
src/test/regress/expected/subselect.out |   11 +++++++++++
src/test/regress/sql/subselect.sql      |    7 +++++++
3 files changed, 40 insertions(+), 5 deletions(-)