[wasm] Handle no initial memory case correctly when memory is exported

test/mjsunit/regress/wasm/regression-710844.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");
+
+(function() {
+  "use asm";
+  var builder = new WasmModuleBuilder();
+  builder.addMemory(0, 5, true);
+  builder.addFunction("regression_710844", kSig_v_v)
+    .addBody([
+        kExprI32Const, 0x03,
+        kExprNop,
+        kExprGrowMemory, 0x00,
+        kExprI32Const, 0x13,
+        kExprNop,
+        kExprI32StoreMem8, 0x00, 0x10
+        ]).exportFunc();
+ let instance = builder.instantiate();
+ instance.exports.regression_710844();
+})();