[wasm] Add regression test

The regression is already fixed. This just adds a regression test to
ensure it will never be reintroduced.

R=ahaas@chromium.org
BUG=chromium:729991

Change-Id: I5cf960cc756cbb7723041bc06a78d6a14c66e241
Reviewed-on: https://chromium-review.googlesource.com/525538Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45739}

test/mjsunit/regress/wasm/regression-729991.js

+// Copyright 2017 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.
+
+load('test/mjsunit/wasm/wasm-constants.js');
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+let builder = new WasmModuleBuilder();
+builder.addCustomSection('BBBB', []);
+builder.addCustomSection('AAAA', new Array(32).fill(0));
+let buffer = builder.toBuffer();
+// Shrink the buffer by 30 bytes (content of the unknown section named 'AAAA').
+buffer = buffer.slice(0, buffer.byteLength - 30);
+// Instantiation should fail on the truncated buffer.
+assertThrows(() => new WebAssembly.Module(buffer), WebAssembly.CompileError);