pgsql: Fix off-by-one Asserts in FreePageBtreeInsertInternal/Leaf.

Fix off-by-one Asserts in FreePageBtreeInsertInternal/Leaf.

These two functions expect there to be room to insert another item
in the FreePageBtree's array, but their assertions were too weak
to guarantee that. This has little practical effect granting that
the callers are not buggy, but it seems to be misleading late-model
Coverity into complaining about possible array overrun.

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/799984.1761150474@sss.pgh.pa.us
Backpatch-through: 13

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7b542cdbcdcc8676c38ebac940d81fa808081681

Modified Files
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src/backend/utils/mmgr/freepage.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)