reflect: fix StructOf GC programs

They are missing a stop byte at the end.

Normally this doesn't matter, but when including a GC program
in another GC program, we strip the last byte. If that last byte
wasn't a stop byte, then we've thrown away part of the program
we actually need.

Fixes #30606

Change-Id: Ie9604beeb84f7f9442e77d31fe64c374ca132cce
Reviewed-on: https://go-review.googlesource.com/c/go/+/165857
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/type.go

@@ -2712,6 +2712,7 @@ func StructOf(fields []StructField) Type {
 				}
 			}
 		}
+		prog = append(prog, 0)
 		*(*uint32)(unsafe.Pointer(&prog[0])) = uint32(len(prog) - 4)
 		typ.kind |= kindGCProg
 		typ.gcdata = &prog[0]

test/fixedbugs/issue30606.go

+// run
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "reflect"
+
+func main() {}
+
+func typ(x interface{}) reflect.Type { return reflect.ValueOf(x).Type() }
+
+var x = reflect.New(reflect.StructOf([]reflect.StructField{
+	{Name: "F5", Type: reflect.StructOf([]reflect.StructField{
+		{Name: "F4", Type: reflect.ArrayOf(5462,
+			reflect.SliceOf(typ(uint64(0))))},
+	})},
+}))