[sanitizer] Change Mmap*NoAccess to return nullptr on error

Summary:
`MmapNoAccess` & `MmapFixedNoAccess` return directly the result of
`internal_mmap`, as opposed to other Mmap functions that return nullptr.

This inconsistency leads to some confusion for the callers, as some check for
`~(uptr)0` (`MAP_FAILED`) for failure (while it can fail with `-ENOMEM` for
example).

Two potential solutions: change the callers, or make the functions return
`nullptr` on failure to follow the precedent set by the other functions.
The second option looked more appropriate to me.

Correct the callers that were wrongly checking for `~(uptr)0` or
`MAP_FAILED`.

TODO for follow up CLs:
- There are a couple of `internal_mmap` calls in XRay that check for
  MMAP_FAILED as a result as well (cc: @dberris); they should use
  `internal_iserror`;


Reviewers: eugenis, alekseyshl, dberris, kubamracek

Reviewed By: alekseyshl

Subscribers: kristina, kubamracek, delcypher, #sanitizers, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D50940

llvm-svn: 340576