DynamicMemory::end() causes "vector iterator not dereferencable"

[BenHanson]

Changing it to:

	const PositionPair *end() const {
		return data.empty() ? nullptr : &data.back() + 1;
	}

works fine.

EDIT:

Also change begin() to check for empty vector:

const PositionPair * begin() const {
	return data.empty() ? nullptr : &data.front();
}