[wasm] Test and fix for module with no functions

Initialize the code table with a valid default (e.g. illegal builtin),
otherwise we're invalidating assumptions when relocating.

Bug: chromium:757217
Change-Id: I77890f1fe0e31534d9844d2e91694df1ec185110
Reviewed-on: https://chromium-review.googlesource.com/630097Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47560}

src/wasm/module-compiler.cc

@@ -2094,7 +2094,9 @@ class AsyncCompileJob::PrepareAndStartCompile : public CompileStep {
     int code_table_size = static_cast<int>(module_->functions.size() +
                                            module_->num_exported_functions);
     job_->code_table_ = factory->NewFixedArray(code_table_size, TENURED);
-
+    for (int i = 0, e = module_->num_imported_functions; i < e; ++i) {
+      job_->code_table_->set(i, *illegal_builtin);
+    }
     // Transfer ownership of the {WasmModule} to the {ModuleCompiler}, but
     // keep a pointer.
     WasmModule* module = module_.get();

test/mjsunit/regress/wasm/regression-757217.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.addImport('','f', kSig_v_v);
+builder.addExport('a', 0);
+builder.addExport('b', 0);
+var bytes = builder.toBuffer();
+
+var m = new WebAssembly.Module(bytes);
+assertTrue(m instanceof WebAssembly.Module);
+
+assertPromiseResult(
+  WebAssembly.compile(bytes)
+  .then(async_result => assertTrue(async_result instanceof WebAssembly.Module),
+        assertUnreachable));