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

[Sergey Matveev]

  I see.
  We can change this just for PointerIsMine, like so:

    bool PointerIsMine(void *p) {
      void *block_beg = GetBlockBegin(p);
      return block_beg != 0 && p >= block_beg;
    }

  But I'm totally fine with moving this check to lsan code.

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