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.