Fix memory leak in pgbench

Fix memory leak in pgbench

Commit 25ee70511ec2 introduced a memory leak in pgbench: some PGresult
structs were not being freed during error bailout, because we're now
doing more PQgetResult() calls than previously.  Since there's more
cleanup code outside the discard_response() routine than in it, refactor
the cleanup code, removing the routine.

This has little effect currently, since we abandon processing after
hitting errors, but if we ever get further pgbench features (such as
testing for serializable transactions), it'll matter.

Per Coverity.

Reviewed-by: Michaël Paquier