Fix NullPointerException in Annotations#iterator

If the annotations `Map` hasn't yet been initialized, initialize it
instead of throwing an NPE.

src/main/java/com/fasterxml/classmate/Annotations.java

@@ -67,6 +67,9 @@ public void addAsDefault(Annotation defValue)
 
     public Iterator<Annotation> iterator()
     {
+        if (_annotations == null) {
+            _annotations = new HashMap<Class<? extends Annotation>,Annotation>();
+        }
         return _annotations.values().iterator();
     }