pgbench - very minor bug fix on hash() missing argument

While doing something else, I noticed that pgbench's hash() does not fail 
gracefully:

  sh> cat hash.sql
  \set i hash()

  sh> pgbench -f hash.sql -t 1
  ...
  cannot coerce (null) to int
  client 0 aborted in command 0 (set) of script 0; evaluation of meta-command failed

The message is not very helful. With the attached one-line patch plus 
test, it is clearer:

  sh> pgbench -f hash.sql -t 1
  hash.sql:1: unexpected number of arguments (hash) in command "set"
  \set i hash()
              ^ error found here

Could be backpatched to 11 where hash was introduced.

-- 
Fabien.