anv/allocator: Move the alignment assert for the pointer free list

anv/allocator: Move the alignment assert for the pointer free list

Previously we asserted every time you tried to pack a pointer and a counter
together.  However, this wasn't really correct.  In the case where you try
to grab the last element of the list, the "next elemnet" value you get may
be bogus if someonoe else got there first.  This was leading to assertion
failures even though the allocator would safely fall through to the failure
case below.