Fix integer-overflow problem in scram_SaltedPassword()

Fix integer-overflow problem in scram_SaltedPassword()

Setting the iteration count for SCRAM secret generation to INT_MAX
will cause an infinite loop in scram_SaltedPassword() due to integer
overflow, as the loop uses the "i <= iterations" comparison.  To fix,
use "i < iterations" instead.

Back-patch to v16 where the user-settable GUC scram_iterations has
been added.

Author: Kevin K Biju <kevinkbiju@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: http://postgr.es/m/CAM45KeEMm8hnxdTOxA98qhfZ9CzGDdgy3mxgJmy0c+2WwjA6Zg@mail.gmail.com