Fix definition of MiniHeap::bytesFree

[aneeshdurg]

Previously MiniHeap::bytesFree was reporting the bytes allocated, not the bytes free. This was impacting GlobalHeap::allocSmallMiniheaps which was always seeing bytesFree() return 0, and thus was always allocating the maximum number of MiniHeaps instead of stopping at bytesFree >= kMiniheapRefillGoalSize