Eng 7004 strict on memory cleanup

It's good that the ugly code in this block is now only on one place instead of two, but it still only differs by one argument ("m_allocationSize" vs. "nexthigher(size)") from two more code blocks in this function.
Factor this block out into an inline addChunk(uint64_t m_allocationSize) method so the ugly code can be in one place instead of three.