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 31966b1.

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-

Author: anarazel

src/backend/storage/buffer/bufmgr.c

@@ -1767,7 +1767,7 @@ LimitAdditionalPins(uint32 *additional_pins)
 	 */
 	max_proportional_pins -= PrivateRefCountOverflowed + REFCOUNT_ARRAY_ENTRIES;
 
-	if (max_proportional_pins < 0)
+	if (max_proportional_pins <= 0)
 		max_proportional_pins = 1;
 
 	if (*additional_pins > max_proportional_pins)