gc: zero stack-allocated slice backing arrays

Fixes Han-Wen's termite bug.

R=lvd
CC=golang-dev
https://golang.org/cl/4977052

src/cmd/gc/sinit.c

@@ -696,6 +696,7 @@ slicelit(int ctxt, Node *n, Node *var, NodeList **init)
 	if(n->esc == EscNone) {
 		a = nod(OXXX, N, N);
 		tempname(a, t);
+		*init = list(*init, nod(OAS, a, N));  // zero new temp
 		a = nod(OADDR, a, N);
 	} else {
 		a = nod(ONEW, N, N);

test/escape3.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+// Test run-time behavior of escape analysis-related optimizations.
+
+package main
+
+func main() {
+	test1()
+}
+
+func test1() {
+	check1(0)
+	check1(1)
+	check1(2)
+}
+
+type T1 struct {
+	X, Y, Z int
+}
+
+func f() int {
+	return 1
+}
+
+func check1(pass int) T1 {
+	v := []T1{{X: f(), Z: f()}}
+	if v[0].Y != 0 {
+		panic("nonzero init")
+	}
+	v[0].Y = pass
+	return v[0]
+}