Protect against partially initialized executer workers

The registerJobs method of an executorworker can be invoked by an
external caller before the worker is completely initialized by
its run method.  We protected against that by checking one instance
variable, but there's still a race condition involving another.
Add a check for that variable as well.

Change-Id: I8e2cfffb54aa8a4cf8b1e61e9a9184b091054462

src/main/java/hudson/plugins/gearman/ExecutorWorkerThread.java

@@ -107,7 +107,7 @@ public class ExecutorWorkerThread extends AbstractWorkerThread{
      */
     @Override
     public void registerJobs() {
-        if (worker == null) {
+        if (worker == null || functionMap == null) {
             // We haven't been initialized yet; the run method will call this again
             return;
         }