* Interpreter tried to access a nonexistent matrix for external reference.
* In ScInterpreter::PopExternalDoubleRef(ScMatrixRef& rMat) set error for
  unexpected conditions. In caller or descendants check for error or presence
  of ScMatrix.

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.