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

src/pkg/container/list/list.go

@@ -29,7 +29,7 @@ type Element struct {
 
 // Next returns the next list element or nil.
 func (e *Element) Next() *Element {
-	if p := e.next; p != &e.list.root {
+	if p := e.next; e.list != nil && p != &e.list.root {
 		return p
 	}
 	return nil
@@ -37,7 +37,7 @@ func (e *Element) Next() *Element {
 
 // Prev returns the previous list element or nil.
 func (e *Element) Prev() *Element {
-	if p := e.prev; p != &e.list.root {
+	if p := e.prev; e.list != nil && p != &e.list.root {
 		return p
 	}
 	return nil

src/pkg/container/list/list_test.go

@@ -234,6 +234,24 @@ func TestIssue4103(t *testing.T) {
 	}
 }
 
+func TestIssue6349(t *testing.T) {
+	l := New()
+	l.PushBack(1)
+	l.PushBack(2)
+
+	e := l.Front()
+	l.Remove(e)
+	if e.Value != 1 {
+		t.Errorf("e.value = %d, want 1", e.Value)
+	}
+	if e.Next() != nil {
+		t.Errorf("e.Next() != nil")
+	}
+	if e.Prev() != nil {
+		t.Errorf("e.Prev() != nil")
+	}
+}
+
 func TestMove(t *testing.T) {
 	l := New()
 	e1 := l.PushBack(1)