pgsql: Fix division by zero in _bt_vacuum_needs_cleanup()

Fix division by zero in _bt_vacuum_needs_cleanup()

Checks inside _bt_vacuum_needs_cleanup() allow division by zero to happen when
metad->btm_last_cleanup_num_heap_tuples == 0.  This commit adjusts the
expression so that no division by zero might happen.

Reported-by: Piotr Stefaniak
Discussion: https://postgr.es/m/DB8PR03MB5931C41F7787A95313F08322F22A0%40DB8PR03MB5931.eurprd03.prod.outlook.com
Reviewed-by: Masahiko Sawada
Backpatch-through: 11

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0d68ad3fc28377d36a3e1dd8b0e2ff47e693e552

Modified Files
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src/backend/access/nbtree/nbtree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)