This is a copy of https://golang.org/cl/23780 for the x/sys repo.

Don't panic, crash, or return references to uninitialized memory when 
ParseDirent is passed invalid input.

Updates golang/go#15653
Fixes golang/go#19754

Change-Id: Idb7cffe14d48ed662e5a55ecb5249c1907cf4003
Reviewed-on: https://go-review.googlesource.com/38758Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>