fixed error when logging from the clustered master

lib/layouts.js

@@ -213,7 +213,7 @@ function patternLayout (pattern, tokens) {
   }
 
   function pid(loggingEvent) {
-    if (loggingEvent.pid) {
+    if (loggingEvent && loggingEvent.pid) {
       return loggingEvent.pid;
     } else {
       return process.pid;