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

test/mjsunit/regress/regress-581.js

 // Copyright 2013 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
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 //
 //     * Redistributions of source code must retain the above copyright
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@@ skipping 18 matching lines @@
 var pow31 = Math.pow(2, 31);
 
 var a = [];
 a[pow31] = 31;
 
 assertEquals(pow31 + 1, a.length);
 assertThrows(function() { a.concat(a); }, RangeError);
 
 var b = [];
 b[pow31 - 3] = 32;
-b[pow31 - 2] = "out_of_bounds";
 var ab = a.concat(b);
 assertEquals(2 * pow31 - 1, ab.length);
 assertEquals(31, ab[pow31]);
 assertEquals(32, ab[2 * pow31 - 2]);
 assertEquals(undefined, ab[2 * pow31 - 1]);
 
 var c = [];
 c[pow30] = 30;
 assertThrows(function() { c.concat(c, a); }, RangeError);