pgbench: Fix overflow in table populating when rows >= 2^31-1

pgbench: Fix overflow in table populating when rows >= 2^31-1

Using a scale factor large enough so as the number of rows to insert
gets larger than INT32_MAX would cause an infinite loop in
initPopulateTable(), preventing pgbench to finish its initialization.

Oversight in e35cc3b3f2d0 that has refactored the data generation logic.

Author: John Hsu
Reviewed-by: Tatsuo Ishii, Japin Li
Discussion: https://postgr.es/m/CA+-JvFvHsOafjHcuFPfkyouHNZvbOXhBNhwZxKm3WNgYz9bwzA@mail.gmail.com