fixed an NPE regression I introduced while modifying the patch.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@20900 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/Hudson.java

@@ -1958,7 +1958,7 @@ public final class Hudson extends Node implements ItemGroup<TopLevelItem>, Stapl
      * @see hudson.tasks.LabelFinder
      */
     public Set<Label> getDynamicLabels() {
-        if (dynamicLabels.isChanged(toComputer()))
+        if (dynamicLabels==null || dynamicLabels.isChanged(toComputer()))
             // in the worst cast, two threads end up doing the same computation
             // twice, but that won't break the semantics.
             // OTOH, not locking prevents dead-lock. See #1390

core/src/main/java/hudson/model/Slave.java

@@ -244,7 +244,7 @@ public abstract class Slave extends Node implements Serializable {
         // so a care needs to be taken to avoid race conditions under all 
         // circumstances.
         synchronized (this) {
-            if (dynamicLabels.isChanged(toComputer()))
+            if (dynamicLabels==null || dynamicLabels.isChanged(toComputer()))
                 dynamicLabels = new DynamicLabels(toComputer());
         }
         return dynamicLabels.labels;