partial rune at end makes string range loop forever

R=rsc
DELTA=20  (20 added, 0 deleted, 0 changed)
OCL=34330
CL=34332

test/bugs/bug204.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 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
+
+func main() {
+	nchar := 0;
+	a := []int { '日', '本', '語', 0xFFFD };
+	for pos, char := range "日本語\xc0" {
+		if nchar >= len(a) {
+			println("BUG");
+			break;
+		}
+		if char != a[nchar] {
+			println("expected", a[nchar], "got", char);
+			println("BUG");
+			break;
+		}
+		nchar++;
+	}
+}