Fix for Issue #174 (exception seen in Travis)

The issue is documented here:
#174

The variable 'worker' being accessed at line# 279 is being obtained
using a look-up into the 'workers' variable where the key to lookup
is coming from '_workers'. It seems '_workers' and 'workers' are not
in sync, and the key obtained from '_workers' does not exist in
'workers' causing the exception. Though any race conditions causing
this being identified and eliminated should be the ideal fix; however,
it was also seen that extending the check at line# 279 from
'if(worker.launched)' to 'if(!worker || worker.launched)' does not
cause any side effect on the code following this if block, because
all this if block does is to return (stop processing this 'worker').
Ideally, if 'worker' does not even exist, that is equally good reason
to stop processing as well, and that's my logic for the statement that
'no side effects are caused'.

Author: reeteshranjan

bin/cli.js

@@ -276,7 +276,7 @@ var statusPoller = {
         }).forEach(function(_worker) {
           var workerData = workerKeys[_worker.id];
           var worker = workers[workerData.key];
-          if (worker.launched) {
+          if (!worker || worker.launched) {
             return;
           }