[BUG] Crash on pgbench initialization.

Hello!

My colleague Victoria Shepard reported that pgbench might crash
during initialization with some values of shared_buffers and
max_worker_processes in conf.

After some research, found this happens when the LimitAdditionalPins() returns exactly zero.
In the current master, this will happen e.g. if shared_buffers = 10MB and max_worker_processes = 40.
Then the command "pgbench --initialize postgres" will lead to crash.
See the backtrace attached.

There is a fix in the patch applied. Please take a look on it.

With the best regards,

-- 
Anton A. Melnikov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company