Don't attempt to dereference basep if it's nil, just pass it to
getdirentries_freebsd12 as is.

Ported from x/sys/unix CL 183223

Change-Id: Id1c4e0eb6ff36dd39524da8194fed9a5957bce61
Reviewed-on: https://go-review.googlesource.com/c/go/+/183797Reviewed-by: Ian Lance Taylor <iant@golang.org>