pgsql: Fix another empty nbtree index SSI race.

Fix another empty nbtree index SSI race.

Commit f9b7fc65 fixed a race when predicate-locking completely empty
btrees: without a buffer lock held, a matching key could be inserted
between _bt_search and the PredicateLockRelation call, so the scan would
miss concurrently inserted tuples while the writer wouldn't see the
reader's predicate lock. That commit only fixed _bt_first's _bt_search
path, though. Scans without useful insertion scan keys return early
from _bt_first via _bt_endpoint, which still didn't recheck if the
relation was empty.

To fix, add handling to _bt_endpoint that is analogous to the handling
added to _bt_search by commit f9b7fc65.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://postgr.es/m/CAH2-WzkNoTn3yXY0iGkSuavJ+sL8EROf+kitW+_2v2tJVWuKmA@mail.gmail.com
Backpatch-through: 14

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2aa3c6d1fceecd94b1f605f0c62884cb686e67ed

Modified Files
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src/backend/access/nbtree/nbtsearch.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)