Make sure Index and IndexByte don't read past the queried byte slice.

Hopefully will be helpful for CL 33597.

Also remove the code which maps/unmaps the Go heap.
Much safer to play with protection bits off-heap.

Change-Id: I50d73e879b2d83285e1bc7c3e810efe4c245fe75
Reviewed-on: https://go-review.googlesource.com/75890Reviewed-by: Ian Lance Taylor <iant@golang.org>