I can't see that nCutPos = rInf.GetLen() makes a lot of sense, perhaps nCutPos
= rInf.GetIdx or nCutPos = rInf.GetLen() + rInf.GetIdx() makes more sense, but
nCutPos is only used in SwTxtPortion::BreakCut and the nLen has to be >= 0 to
be useful in there, so might as well straight-out flat invalidate it as -1 and
test for positive values in SwTxtPortion::BreakCut

Change-Id: Iad0c26efc47990cf2ffed54dc91207c3b8551096