runtime: fix a panic when growing zero-width-element slices.

Fixes #4197.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6611056

src/pkg/runtime/slice.c

@@ -114,7 +114,7 @@ runtime·growslice(SliceType *t, Slice old, int64 n, Slice ret)
 
 	cap = old.cap + n;
 
-	if((intgo)cap != cap || cap < old.cap || cap > MaxMem / t->elem->size)
+	if((intgo)cap != cap || cap < old.cap || (t->elem->size > 0 && cap > MaxMem/t->elem->size))
 		runtime·panicstring("growslice: cap out of range");
 
 	growslice1(t, old, cap, &ret);

test/fixedbugs/bug457.go

+// run
+
+// Copyright 2012 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 4197: growing a slice of zero-width elements
+// panics on a division by zero.
+
+package main
+
+func main() {
+	var x []struct{}
+	x = append(x, struct{}{})
+}