[eigen] bug in assigning triangularView to MatrixBase

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this time I found a real one. ;)

The code below crashes because
MatrixBase<Derived>::operator=(EigenBase) is directly calling evalTo
without resizing.

I tried to template just a MatrixType and the the code is working
because Matrix<Derived>::operator=(EigenBase) calls
DenseStorageBase::operator=(EigenBase) which does the resizing before
calling MatrixBase<Derived>::operator=(EigenBase).

The question is where do we put the resizing code in order to be sure
we don't run into any trouble again?

I was wondering about putting the resize close to evalTo and did a
quick grep. ReturnByValue will cause the same troubles when it is used
in functions taking MatrixBase<Derived>.

- Hauke


template <typename Derived>
void crash(const MatrixBase<Derived>& M, MatrixBase<Derived>& N)
  HouseholderQR<Derived> qr(M);
  N = qr.matrixQR().triangularView<Upper>();

void main()
  const int size = 50;
  typedef MatrixXd MatrixType;
  MatrixType N,M = MatrixType::Random(size,size);


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