FieldMatrix Assignment/ Construction from DenseMatrix reference returns empty matrix

The use case that triggered the bug was

auto foo () 
{
 // do something
  return (A *=  s);
}

where A is a FieldMatrix and s a scalar value. The returned value is always null. Assignment behaves the same way.

Attached is a patch for fmatrixtest.cc checks the validity of

// test 1
DenseMatrix<decltype(A)>& B = A;
C = B;

// test 2
B = (A*=1);

Additionally the line

DenseMatrix< decltype( A ) >B = A;

is added, which according to @martin.nolte should not even compile.