fix NPE ( From github user：aCoder2013 )

xxl-job-admin/src/main/java/com/xxl/job/admin/service/impl/XxlJobServiceImpl.java

 
 		// stage job info
 		XxlJobInfo exists_jobInfo = xxlJobInfoDao.loadById(jobInfo.getId());
-        String old_cron = exists_jobInfo.getJobCron();
 		if (exists_jobInfo == null) {
 			return new ReturnT<String>(ReturnT.FAIL_CODE, "参数异常");
 		}
+		//String old_cron = exists_jobInfo.getJobCron();
 
 		exists_jobInfo.setJobCron(jobInfo.getJobCron());
 		exists_jobInfo.setJobDesc(jobInfo.getJobDesc());