`grb::mxv<dense>` with sparse output returns `grb::SUCCESS`

[GiovaGa]

When calling grb::mxv() with a dense descriptor but output vector not structurally dense, the return code is grb::SUCCESS instead of the expected grb::ILLEGAL.

For example:

	grb::Vector<float> x( 5 ), y( 5 );
	grb::Matrix<float> A( 5, 5 );
	
	// Build the vectors and the matrix ...
	
	auto plusTimes = grb::semirings::plusTimes< float >();

	grb::clear( x );
	// grb::set( x, 0 ); // without this line x is not initialized and thus not dense
	grb::set( y, 1 ); // without this line x is not initialized and thus not dense

	std::cerr << "grb::nnz(x) = " << grb::nnz(x) << std::endl; // this prints 0

	rc = grb::mxv< grb::descriptors::dense >(x, A, y, plusTimes);
	assert( rc == grb::SUCCESS );

Runs successfully even though it should throw an error.

This behavior has been observed with the following backends: reference, reference_omp, nonblocking, bsp1d, hyperdags, hybrid

[GiovaGa]

This is related to #388