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

src/bin/pgbench/pgbench.c

@@ -4908,7 +4908,7 @@ initPopulateTable(PGconn *con, const char *table, int64 base,
 				  initRowMethod init_row)
 {
 	int			n;
-	int			k;
+	int64		k;
 	int			chars = 0;
 	PGresult   *res;
 	PQExpBufferData sql;