runtime: don't return a slice with nil ptr but non-zero len from growslice

Fixes #10135.

Change-Id: Ic4c5ab15bcb7b9c3fcc685a788d3b59c60c26e1e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7400Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/slice.go

@@ -50,7 +50,9 @@ func growslice(t *slicetype, old sliceStruct, n int) sliceStruct {
 
 	et := t.elem
 	if et.size == 0 {
-		return sliceStruct{old.array, old.len, cap}
+		// append should not create a slice with nil pointer but non-zero len.
+		// We assume that append doesn't need to preserve old.array in this case.
+		return sliceStruct{unsafe.Pointer(&zerobase), old.len, cap}
 	}
 
 	newcap := old.cap

test/fixedbugs/issue10135.go

+// run
+
+// Copyright 2015 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.
+
+// Issue 10135: append a slice with zero-sized element used
+// to always return a slice with the same data pointer as the
+// old slice, even if it's nil, so this program used to panic
+// with nil pointer dereference because after append, s is a
+// slice with nil data pointer but non-zero len and cap.
+
+package main
+
+type empty struct{}
+
+func main() {
+	var s []empty
+
+	s = append(s, empty{})
+
+	for _, v := range s {
+		_ = v
+	}
+}