pgsql: Fix amcheck's handling of half-dead B-tree pages

Fix amcheck's handling of half-dead B-tree pages

amcheck incorrectly reported the following error if there were any
half-dead pages in the index:

ERROR:  mismatch between parent key and child high key in index
"amchecktest_id_idx"

It's expected that a half-dead page does not have a downlink in the
parent level, so skip the test.

Reported-by: Konstantin Knizhnik <knizhnik@garret.ru>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Discussion: https://www.postgresql.org/message-id/33e39552-6a2a-46f3-8b34-3f9f8004451f@garret.ru
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7792bdc45860ad9f0e7188ce2113f799a074034d

Modified Files
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contrib/amcheck/verify_nbtree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)