Infinite loop in pgbench when running COPY command

Hi,

Currently, pgbench processes a copy response as unexpected and will
move to the error loop. However, PQgetResult will alway return an
empty result when there's no async result through getCopyResult,
leading to an infinite loop in the error handling as res will never be
NULL.

This patch forcefully exits the copy state with PQendcopy before
moving to the error handler, avoiding the infinite loop.

Regards,
Anthonin Bonnefoy