[wasm] Fix check failure on invalid name section

test/mjsunit/regress/wasm/regression-684858.js

Index: test/mjsunit/regress/wasm/regression-684858.js
diff --git a/test/mjsunit/regress/wasm/regression-684858.js b/test/mjsunit/regress/wasm/regression-684858.js
new file mode 100644
index 0000000000000000000000000000000000000000..e11d909c8621e3a463d11406b12ae084af887af3
--- /dev/null
+++ b/test/mjsunit/regress/wasm/regression-684858.js
@@ -0,0 +1,34 @@
+// 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');
+
+var name = 'regression_684858';
+
+function patchNameLength(buffer) {
+  var count = 0;
+  var view = new Uint8Array(buffer);
+  for (var i = 0, e = view.length - name.length; i < e; ++i) {
+    var subs = String.fromCharCode.apply(null, view.slice(i, i + name.length));
+    if (subs != name) continue;
+    ++count;
+    // One byte before this name, its length is encoded.
+    // Patch this to 127, making it out of bounds.
+    if (view.length >= 127) throw Error('cannot patch reliably');
+    if (view[i - 1] != name.length) throw Error('unexpected length');
+    view[i - 1] = 0x7f;
+  }
+  if (count != 1) throw Error('did not find name');
+}
+
+var builder = new WasmModuleBuilder();
+builder.addFunction(name, kSig_i_v)
+    .addBody([kExprI32Const, 2, kExprI32Const, 0, kExprI32DivU])
+    .exportAs('main');
+var buffer = builder.toBuffer();
+patchNameLength(buffer);
+var module = new WebAssembly.Module(buffer);
+var instance = new WebAssembly.Instance(module);
+assertThrows(() => instance.exports.main(), WebAssembly.RuntimeError);