Check for null subclassNode

Author: headius

core/src/main/java/org/jruby/RubyClass.java

@@ -1104,6 +1104,8 @@ private void finishSubclasses(Collection<RubyClass> mine, int clearedCount, bool
     private int concreteSubclasses(RubyArray<RubyClass> subs) {
         SubclassNode subclassNode = this.subclassNode;
 
+        if (subclassNode == null) return 0;
+
         // skip first entry if not concrete
         if (!subclassNode.concrete) subclassNode = subclassNode.nextConcrete;