pgsql: Fix _bt_readnextpage half-dead leftmost page assert.

Fix _bt_readnextpage half-dead leftmost page assert.

Backwards scans that reach a half-dead leftmost leaf page step over the
page without calling _bt_readpage. This happens whenever the most
recent _bt_lock_and_validate_left call returned a half-dead leftmost
page. An assertion within _bt_readnextpage failed to consider this,
causing it to fail spuriously.

To fix, make the assertion accept the case where currPage doesn't match
lastcurrblkno as long as blkno is P_NONE (it's always P_NONE when the
last leaf page examined by _bt_readnextpage is either the leftmost or
rightmost page).

Oversight in commit caca6d8d27.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Backpatch-through: 18

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6d4ee9203686d0426ec089ce965a96a86ef4651d

Modified Files
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src/backend/access/nbtree/nbtsearch.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)