The problem is that SwIndexReg::Update will correct a 1-character
redline in the middle of the newly-inserted part of the overwrite string
into a 0-length redline, which then a later SwTextNode::Update() will
correct in such a way that the whole thing becomes unsorted.

Just delete redlines in the entire overwrite range, which should help;
the aPam actually deletes them in the *last* character only which
seems rather unintentional anyway.

Change-Id: I61b6b312998e0779651d30f636312ef13556428c