Guard against record.err.stack not being present before assigning to msg

Author: Binarytales

lib/bunyan-stackdriver.js

@@ -93,7 +93,7 @@ BunyanStackDriver.prototype._write = function write(record, encoding, callback)
   record = destroyCircular(record);
   strictJSON(record);
 
-  if (record.err) {
+  if (record.err && record.err.stack) {
       record.message = record.err.stack;
   } else if (!record.message) {
     record.message = record.msg;