Correct job_cnt_run NULL pointer

This corrects logic from commit e5a61746
that could result in use of NULL pointer

src/slurmctld/read_config.c

@@ -1957,7 +1957,8 @@ static int _sync_nodes_to_active_job(struct job_record *job_ptr)
 		}
 	}
 
-	if (IS_JOB_RUNNING(job_ptr) || IS_JOB_SUSPENDED(job_ptr))
+	if ((IS_JOB_RUNNING(job_ptr) || IS_JOB_SUSPENDED(job_ptr)) &&
+	    (job_ptr->front_end_ptr != NULL))
 		job_ptr->front_end_ptr->job_cnt_run++;
 
 	return cnt;