Fix off-by-one in Array.concat's max index check

test/mjsunit/regress/regress-crbug-516592.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var i = Math.pow(2, 31);
+var a = [];
+a[i] = 31;
+var b = [];
+b[i - 2] = 33;
+try {
+  // This is supposed to throw a RangeError.
+  var c = a.concat(b);
+  // If it didn't, ObservableSetLength will detect the problem.
+  Object.observe(c, function() {});
+  c.length = 1;
+} catch(e) {
+  assertTrue(e instanceof RangeError);
+}