update document

xxl-job-core/src/main/java/com/xxl/job/core/thread/JobThread.java

 					if (executeResult == null) {
 						executeResult = IJobHandler.FAIL;
 					} else {
-						executeResult.setMsg((executeResult!=null&&executeResult.getMsg().length()>50000)?executeResult.getMsg().substring(0, 50000).concat("..."):executeResult.getMsg());
+						executeResult.setMsg(
+								(executeResult!=null&&executeResult.getMsg()!=null&&executeResult.getMsg().length()>50000)
+										?executeResult.getMsg().substring(0, 50000).concat("...")
+										:executeResult.getMsg());
 						executeResult.setContent(null);	// limit obj size
 					}
 					XxlJobLogger.log("<br>----------- xxl-job job execute end(finish) -----------<br>----------- ReturnT:" + executeResult);