gc: missed typecheck in subscripting a const string.

Fixes #2674.

R=rsc
CC=golang-dev
https://golang.org/cl/5574045

src/cmd/gc/walk.c

@@ -876,6 +876,7 @@ walkexpr(Node **np, NodeList **init)
 				// delayed until now because "abc"[2] is not
 				// an ideal constant.
 				nodconst(n, n->type, n->left->val.u.sval->s[v]);
+				n->typecheck = 1;
 			}
 		}
 		goto ret;

test/fixedbugs/bug399.go

+// $G $D/$F.go || echo "Bug399"
+
+// 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.
+
+// Issue 2674
+
+package main
+const dow = "\000\003"
+
+func main() {
+	println(int(dow[1]))
+}
+