Fix infinite loop from setting scram_iterations to INT_MAX

Hi,

I stumbled upon a problem with the scram_iterations GUC where setting
scram_iterations to INT_MAX and then creating a user causes the command to
hang indefinitely.

postgres=# SET scram_iterations=2147483647;
SET
postgres=# CREATE ROLE maxscram WITH PASSWORD 'forever';
<hangs>

I looked into the relevant code and found the issue. Each SCRAM iteration
after the first is done in a loop with the following condition:

int i;
...
for (i = 2; i <= iterations; i++)
{
    ...
}

For iterations = INT_MAX, the loop will never terminate since the condition
is <= and adding 1 to INT_MAX will lead to i wrapping around to INT_MIN.

I've fixed this by modifying the loop condition to be i < iterations. I've
attached a patch with the fix. I considered adding a test as well, but
since generating a password with a high number of iterations is very
time-consuming, I'm not sure if that would be practical.

I also tried adding this to the current CommitFest, but my account hasn't
passed the cooldown period yet.

Thanks,
Kevin