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