pgsql: Fix oversight in commit 1ec7fca8592178281cd5cdada0f27a340fb813fc

Fix oversight in commit 1ec7fca8592178281cd5cdada0f27a340fb813fc.

I failed to account for the possibility that when
ExecAppendAsyncEventWait() notifies multiple async-capable nodes using
postgres_fdw, a preceding node might invoke process_pending_request() to
process a pending asynchronous request made by a succeeding node. In
that case the succeeding node should produce a tuple to return to the
parent Append node from tuples fetched by process_pending_request() when
notified. Repair.

Per buildfarm via Michael Paquier. Back-patch to v14, like the previous
commit.

Thanks to Tom Lane for testing.

Discussion: https://postgr.es/m/YQP0UPT8KmPiHTMs%40paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a8ed9bd59d48c13da50ed2358911721b2baf1de3

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 22 ++++++++++++++++++----
src/backend/executor/nodeAppend.c | 22 ++++++++++++----------
2 files changed, 30 insertions(+), 14 deletions(-)