invert_mask descriptor inverts return value to true for unassigned elements

[aleksamilisavljevic]

According to the documentation:

ALP/include/graphblas/utils.hpp

Lines 299 to 307 in 73a3cb3

	* @return If the descriptor does not include grb::descriptors::structural
	* nor grb::descriptors::structural_complement and if \a assigned
	* is false, then the entry is ignored and uninterpreted, thus
	* returning \a false.
	* @return If the descriptor includes grb::descriptors::invert_mask,
	* returns the negation of the dereferenced value of \a val
	* which is first cast to a \a bool.
	* @return Otherwise, returns the dereferenced value of \a val,
	* cast to a \a bool.

However, if the descriptor invert_mask is set and assigned is set to false, then the return value will be true:

ALP/include/graphblas/utils.hpp

Lines 320 to 336 in 73a3cb3

	bool ret = false;
	// if we request a structural mask, decide only on passed assigned variable
	if( descriptor & descriptors::structural ) {
	ret = assigned;
	} else {
	// if based on value, if there is a value, cast it to bool
	if( assigned ) {
	ret = static_cast< bool >( val[ offset ] );
	}
	// otherwise there is no value and false is assumed
	}
	// check whether we should return the inverted value
	if( descriptor & descriptors::invert_mask ) {
	return !ret;
	} else {
	return ret;
	}