container/list: unexpected panic if Next/Prev called outside of list.

container/list: unexpected panic if Next/Prev called outside of list.

Before CL 7065067 calling Next on an element returned either the
next/prev element or nil was returned.  After the CL if an element
was not part of a list e.Next() and e.Prev() will panic.  This CL
returns to the documented behavior, that Next/Prev returns the
next/prev list element or nil.

Fixes #6349.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/13234051