MINOR: log: only run postcheck_log_backend() checks on backend

MINOR: log: only run postcheck_log_backend() checks on backend

postcheck_log_backend() checks are executed no matter if the proxy
actually has the backend capability while the checks actually depend
on this.

Let's fix that by adding an extra condition to ensure that the BE
capability is set.

This issue is not tagged as a bug because for now it remains impossible
to have a syslog proxy without BE capability in the main proxy list, but
this may change in the future.