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/value.go

@@ -1058,7 +1058,12 @@ func (v *PtrValue) SetValue(x Value) {
 }
 
 // PointTo changes v to point to x.
+// If x is a nil Value, PointTo sets v to nil.
 func (v *PtrValue) PointTo(x Value) {
+	if x == nil {
+		*(**uintptr)(v.addr) = nil
+		return
+	}
 	if !x.CanSet() {
 		panic("cannot set x; cannot point to x")
 	}