buffer: Don't assign .parent if none exists

The .parent property of the allocated buffer should remain undefined in
the case that it's not a slice. Also included test to verify this.

PR-URL: https://github.com/iojs/io.js/pull/1109


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>