reflect: fix panic when Value.IsNil is called for UnsafePointer

UnsafePointer is a valid type kind to call IsNil on.

Fixes #29381

Change-Id: Iaf65d582c67f4be52cd1885badf40f174920500b
Reviewed-on: https://go-review.googlesource.com/c/155797
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/reflect/all_test.go

@@ -1009,6 +1009,7 @@ func TestIsNil(t *testing.T) {
 		struct{ x func() bool }{},
 		struct{ x chan int }{},
 		struct{ x []string }{},
+		struct{ x unsafe.Pointer }{},
 	}
 	for _, ts := range doNil {
 		ty := TypeOf(ts).Field(0).Type

src/reflect/value.go

@@ -1031,7 +1031,7 @@ func (v Value) InterfaceData() [2]uintptr {
 func (v Value) IsNil() bool {
 	k := v.kind()
 	switch k {
-	case Chan, Func, Map, Ptr:
+	case Chan, Func, Map, Ptr, UnsafePointer:
 		if v.flag&flagMethod != 0 {
 			return false
 		}