Cleanup node on Crystal::PointerLinkedList#delete (#16398)

Calls to delete, pop or shift a node from the list now clear the `#next` and `#previous` properties of the node.

It's not required per-se, but cleaning up pointers is never a bad idea.

Author: ysbaddaden

spec/std/crystal/pointer_linked_list_spec.cr

@@ -87,6 +87,8 @@ describe Crystal::PointerLinkedList do
       list.push pointerof(w)
 
       list.delete pointerof(y)
+      y.next.should eq(Pointer(TestedObject).null)
+      y.previous.should eq(Pointer(TestedObject).null)
 
       ExpectOrderHelper.by_next(x, z, w, x)
       ExpectOrderHelper.by_previous(x, w, z, x)
@@ -113,6 +115,8 @@ describe Crystal::PointerLinkedList do
 
       obj.should_not be_nil
       obj.should eq(pointerof(x))
+      obj.not_nil!.value.next.should eq(Pointer(TestedObject).null)
+      obj.not_nil!.value.previous.should eq(Pointer(TestedObject).null)
 
       ExpectOrderHelper.by_next(y, z, w, y)
       ExpectOrderHelper.by_previous(y, w, z, y)
@@ -147,6 +151,8 @@ describe Crystal::PointerLinkedList do
 
       obj.should_not be_nil
       obj.should eq(pointerof(w))
+      obj.not_nil!.value.next.should eq(Pointer(TestedObject).null)
+      obj.not_nil!.value.previous.should eq(Pointer(TestedObject).null)
 
       ExpectOrderHelper.by_next(x, y, z, x)
       ExpectOrderHelper.by_previous(x, z, y, x)

src/crystal/pointer_linked_list.cr

@@ -63,6 +63,9 @@ struct Crystal::PointerLinkedList(T)
     else
       @head = Pointer(T).null
     end
+
+    node.value.next = Pointer(T).null
+    node.value.previous = Pointer(T).null
   end
 
   # Removes and returns head from the list, yields if empty