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

test/mjsunit/regress/wasm/regression-724972.js

Index: test/mjsunit/regress/wasm/regression-724972.js
diff --git a/test/mjsunit/regress/wasm/regression-724972.js b/test/mjsunit/regress/wasm/regression-724972.js
new file mode 100644
index 0000000000000000000000000000000000000000..2af403ce202bac5607a2ba8832b8f17a562d58e7
--- /dev/null
+++ b/test/mjsunit/regress/wasm/regression-724972.js
@@ -0,0 +1,11 @@
+// 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 builder = new WasmModuleBuilder();
+builder.addMemory(0, 0, true);
+var instance = builder.instantiate();
+instance.exports.memory.buffer;