Fix crash in postgres_fdw for provably-empty remote UPDATE/DELETE.

In 86dc90056, I'd written find_modifytable_subplan with the assumption
that if the immediate child of a ModifyTable is a Result, it must be
a projecting Result with a subplan.  However, if the UPDATE or DELETE
has a provably-constant-false WHERE clause, that's not so: we'll
generate a dummy subplan with a childless Result.  Add the missing
null-check so we don't crash on such cases.

Per report from Alexander Pyhalov.

Discussion: https://postgr.es/m/[email protected]

0 files changed, 0 insertions, 0 deletions