SwUndoTblNdsChg::UndoImpl: to prevent access of deleted table box start
node, disconnect the SwTableBox from the start node before removing the
table box nodes.
This problem was probably introduced with CWS swnewtable
db4de0817d, which changed the order of the
removal operations for unknown reasons.
Change-Id: Ic59823a84082cc6ff453b2b512cbb8253886ecf3