Empty resources

[andrews05]

I'm sure there's a good reason for writing it this way but this code is currently causing problems when loading zero-length resources. (The resource ends up being the size of the entire file)

Graphite/libGraphite/data/data.cpp

Lines 56 to 67 in e850681

	auto graphite::data::data::size() const -> std::size_t
	{
	if (m_size > 0) {
	return m_size;
	}
	else if (m_data != nullptr) {
	return m_data->size();
	}
	else {
	return 0;
	}
	}