Modestly improve pgbench's checking for invalid ranges.

The old check against MAX_RANDOM_VALUE is clearly irrelevant since
getrand() no longer calls random().  Instead, check whether min and max
are close enough together to avoid an overflow inside getrand(), as
suggested by Tom Lane.  This is still somewhat silly, because we're
using atoi(), which doesn't check for overflow anyway and (at least on
my system) will cheerfully return 0 when given "4294967296".  But that's
a problem for another commit.