The node is moved between undo-array and doc-array and each time
AddToList is called; it doesn't make sense to add a node that is
currently in undo-array to a list, and it leaks the mpNodeNum
because IsInList will return false because the SwNodeNum lacks a
parent, and it triggers some recently added asserts,
so just don't do that.

Change-Id: I75e51386806ce3845b7c61206020a59c092577fe