pgsql: Guard against core dump from uninitialized subplan.

Guard against core dump from uninitialized subplan.

If the planner erroneously puts a non-parallel-safe SubPlan into
a parallelized portion of the query tree, nodeSubplan.c will fail
in the worker processes because it finds a null in es_subplanstates,
which it's unable to cope with.  It seems worth a test-and-elog to
make that an error case rather than a core dump case.

This probably should have been included in commit 16ebab688, which
was responsible for allowing nulls to appear in es_subplanstates
to begin with.  So, back-patch to v10 where that came in.

Discussion: https://postgr.es/m/924226.1604422326@sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/936043c9eacb9e9c7356a8190a410d2c4e4ea03a

Modified Files
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src/backend/executor/nodeSubplan.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)