[PATCH] [sanitizer] Fix overflow in SizeClassAllocator64::GetChunkIdx().

[Sergey Matveev]

  We have about 600 lines of allocator tests, which is far too little.

  Regarding this specific change, you can't test it without allocating 4gb of memory (well, you can if you just feed the hand-crafted pointer to GetBlockBegin - but that's a bit hacky), and the change itself is so trivial that I don't think it's worth adding a regression test.

http://llvm-reviews.chandlerc.com/D797