sw: replace pointless unused position update in SwTxtNode::Update()
sw: replace pointless unused position update in SwTxtNode::Update()

... with a more useful assertion.

Change-Id: I43f0a3e123d9dc9f9fd9548e84b8619c7d4f1262

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core