pgsql: Fix yet another crash in page split during GiST index creation.

Fix yet another crash in page split during GiST index creation.

Commit a7ee7c8513 fixed a bug in GiST page split during index creation,
where we failed to re-find the position of a downlink after the page
containing it was split. However, that fix was incomplete; the other call
to gistinserttuples() in the same function needs to also clear
'downlinkoffnum'.

Fixes bug #16134 reported by Alexander Lakhin, for real this time. The
previous fix was enough to fix the crash with the reproducer script for
bug #16162, but the original script for #16134 was still crashing.

Backpatch to v12, like the previous incomplete fix.

Discussion: https://www.postgresql.org/message-id/d869f537-abe4-d2ea-0510-38cd053f5152%40gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/741b884353e4803abc15d4392ad287b0d5953fc4

Modified Files
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src/backend/access/gist/gist.c | 31 +++++++++++++++++++------------
1 file changed, 19 insertions(+), 12 deletions(-)