It's not sensible to re-evaluate a direct-modify Foreign Update or Delete
during EvalPlanQual. However, ExecInitForeignScan() can still get called
if a table mixes local and foreign partitions. EvalPlanQualStart() left
the es_result_relations array uninitialized in the child EPQ EState, but
ExecInitForeignScan() still expected to find it. That caused a segfault.

Fix by skipping the es_result_relations lookup during EvalPlanQual
processing. To make things a bit more robust, also skip the
BeginDirectModify calls, and add a runtime check that ExecForeignScan()
is not called on direct-modify foreign scans during EvalPlanQual
processing.

This is new in v14, commit 1375422c. Before that, EvalPlanQualStart()
copied the whole ResultRelInfo array to the EPQ EState. Backpatch to v14.

Report and diagnosis by Andrey Lepikhov.

Discussion: https://www.postgresql.org/message-id/cb2b808d-cbaa-4772-76ee-c8809bafcf3d%40postgrespro.ru