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

[Kostya Serebryany]

  What you say justifies the check
    if (h->map_beg + h->map_size <= p)
  I am totally ok with this check, but I ask a test for it.

  asan relies on the absence of your second check:
    if (h->map_beg + page_size_ > p)
  i.e. it expects that [map_beg, map_beg+4095] in in the block.
  you could argue about lack of clear defined interface, blah, blah.
  But I've already added an asan list test to check for this, so please don't break it :)

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