reflect: allow PtrValue.PointTo(nil)

(Argument: For any *PtrValue p, it should
always be possible to do: p.PointTo(p.Elem()),
even if p.Elem() is nil.)

Fixes #1028.

R=rsc
CC=golang-dev, r
https://golang.org/cl/1938044

src/pkg/reflect/all_test.go

@@ -384,6 +384,13 @@ func TestPtrPointTo(t *testing.T) {
 	if *ip != 1234 {
 		t.Errorf("got %d, want 1234", *ip)
 	}
+
+	ip = nil
+	vp := NewValue(ip).(*PtrValue)
+	vp.PointTo(vp.Elem())
+	if ip != nil {
+		t.Errorf("got non-nil (%p), want nil", ip)
+	}
 }
 
 func TestPtrSetNil(t *testing.T) {