[wasm] Fix validation error for missing return statement in asm.js module.

R=mstarzinger@chromium.org,bradnelson@chromium.org
LOG=Y
BUG=chromium:575364

Review URL: https://codereview.chromium.org/1564313003

Cr-Commit-Position: refs/heads/master@{#33175}

src/typing-asm.cc

@@ -127,6 +127,9 @@ void AsmTyper::VisitAsmModule(FunctionLiteral* fun) {
 
   // Validate exports.
   ReturnStatement* stmt = fun->body()->last()->AsReturnStatement();
+  if (stmt == nullptr) {
+    FAIL(fun->body()->last(), "last statement in module is not a return");
+  }
   RECURSE(VisitWithExpectation(stmt->expression(), Type::Object(),
                                "expected object export"));
 }

test/mjsunit/regress/regress-575364.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.
+
+// Flags:  --expose-wasm
+
+function f() {
+  "use asm";
+
+}
+assertThrows(function() { WASM.asmCompileRun(f.toString()); });