fix: DJPP is more tolerant to inconsistent model (#1903)

Author: monperrus

src/main/java/spoon/reflect/visitor/filter/SiblingsFunction.java

@@ -73,7 +73,7 @@ public void apply(final CtElement input, final CtConsumer<Object> outputConsumer
 			boolean visitNext = mode == Mode.ALL || mode == Mode.NEXT;
 			@Override
 			public void scan(CtElement element) {
-				if (element != null && element.getParent() == parent) {
+				if (element != null && element.isParentInitialized() && element.getParent() == parent) {
 					//visit only elements whose parent is same
 					boolean canVisit = hasVisitedInput ? visitNext : visitPrev;
 					if (input == element) {