[wasm] Fix WasmMemoryObject constructor for when a module has no initial memory

test/mjsunit/regress/wasm/regression-727560.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.addMemory();
+  builder.exportMemoryAs("exported_mem");
+  i1 = builder.instantiate();
+}
+{
+  let builder = new WasmModuleBuilder();
+  builder.addImportedMemory("fil", "imported_mem");
+  i2 = builder.instantiate({fil: {imported_mem: i1.exports.exported_mem}});
+}