[release-branch.go1.8] reflect: clear ptrToThis in Ptr when allocating result on heap

Otherwise, calling PtrTo on the result will fail.

Fixes #19003

Change-Id: I8d7d1981a5d0417d5aee52740469d71e90734963
Reviewed-on: https://go-review.googlesource.com/36731
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/36718
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: mwhudson

src/reflect/all_test.go

@@ -2478,17 +2478,24 @@ func TestNumMethodOnDDD(t *testing.T) {
 }
 
 func TestPtrTo(t *testing.T) {
+	// This block of code means that the ptrToThis field of the
+	// reflect data for *unsafe.Pointer is non zero, see
+	// https://golang.org/issue/19003
+	var x unsafe.Pointer
+	var y = &x
+	var z = &y
+
 	var i int
 
-	typ := TypeOf(i)
+	typ := TypeOf(z)
 	for i = 0; i < 100; i++ {
 		typ = PtrTo(typ)
 	}
 	for i = 0; i < 100; i++ {
 		typ = typ.Elem()
 	}
-	if typ != TypeOf(i) {
-		t.Errorf("after 100 PtrTo and Elem, have %s, want %s", typ, TypeOf(i))
+	if typ != TypeOf(z) {
+		t.Errorf("after 100 PtrTo and Elem, have %s, want %s", typ, TypeOf(z))
 	}
 }
 

src/reflect/type.go

@@ -1469,6 +1469,7 @@ func (t *rtype) ptrTo() *rtype {
 	pp := *prototype
 
 	pp.str = resolveReflectName(newName(s, "", "", false))
+	pp.ptrToThis = 0
 
 	// For the type structures linked into the binary, the
 	// compiler provides a good hash of the string.