Overflow hazard in pgbench

moonjelly just reported an interesting failure [1].  It seems that
with the latest bleeding-edge gcc, this code is misoptimized:

                /* check random range */
                if (imin > imax)
                {
                    pg_log_error("empty range given to random");
                    return false;
                }
                else if (imax - imin < 0 || (imax - imin) + 1 < 0)
                {
                    /* prevent int overflows in random functions */
                    pg_log_error("random range is too large");
                    return false;
                }

such that the second if-test doesn't fire.  Now, according to the C99
spec this code is broken, because the compiler is allowed to assume
that signed integer overflow doesn't happen, whereupon the second
if-block is provably unreachable.  The failure still represents a gcc
bug, because we're using -fwrapv which should disable that assumption.
However, not all compilers have that switch, so it'd be better to code
this in a spec-compliant way.  I suggest applying the attached in
branches that have the required functions.

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=moonjelly&dt=2021-06-26%2007%3A03%3A17

			regards, tom lane