pgsql: Fix off-by-one in LimitAdditionalPins()

Fix off-by-one in LimitAdditionalPins()

Due to the bug LimitAdditionalPins() could return 0, violating
LimitAdditionalPins()'s API ("One additional pin is always allowed"). This
could be hit when setting shared_buffers very low and using a fair amount of
concurrency.

This bug was introduced in 31966b151e6a.

Author: "Anton A. Melnikov" <aamelnikov@inbox.ru>
Reported-by: "Anton A. Melnikov" <aamelnikov@inbox.ru>
Reported-by: Victoria Shepard
Discussion: https://postgr.es/m/ae46f2fb-5586-3de0-b54b-1bb0f6410ebd@inbox.ru
Backpatch: 16-

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bd2f46c6559ddcafe7bae5015ac45b69bc014067

Modified Files
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src/backend/storage/buffer/bufmgr.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)