Correctly set previous and next nodes when appending

Author: synthfi

src/mutable_list.jl

@@ -153,7 +153,13 @@ end
 function Base.append!(l1::MutableLinkedList{T}, l2::MutableLinkedList{T}) where T
     l1.node.prev.next = l2.node.next # l1's last's next is now l2's first
     l2.node.prev.next = l1.node # l2's last's next is now l1.node
+    l2.node.next.prev = l1.node.prev # l2's first's prev is now l1's last
+    l1.node.prev      = l2.node.prev # l1's first's prev is now l2's last
     l1.len += length(l2)
+    # make l2 empty
+    l2.node.prev = l2.node
+    l2.node.next = l2.node
+    l2.len = 0
     return l1
 end