pam_systemd: fix error logs

pam_systemd: fix error logs

The code was of two minds about error_id: it was used directly in
pam_syslog_errno(), but in the next line checked with streq_ptr().
sd_varlink_callbo() may return negative and then it does not set the output
params, or it returns the error in ret_error_id. We cannot assume that error_id
is non-null. Also fix a select-and-paste mistake in one place.