When lowering m_nStartPos, do not duplicate rows above its old value

Fixed a misconception in transition from List to to ptr_vector of commit
db599f64, List::Next() returned NULL when at
end of list, mimic that.

This regression was introduced in f2153330.
As stated in PageObjectPainter::PaintPageObject:
  "The page object layouter is quite volatile."
The rest of usages of mpPageObjectLayouter looks sane, but who knows...

Symptom: segfault.
Thanks to Julien Nabet for precise pointer to problematic code.