[PATCH] Fixes in LargeMmapAllocator::GetBlockBegin().

[Sergey Matveev]

  It's causing false negatives in LSan, when an internal data structure points to a block of memory that is mapped immediately after a LargeMmapAllocator chunk.

  Does ASan rely on this behavior in PointerIsMine(), or just GetBlockBegin()?

http://llvm-reviews.chandlerc.com/D624