[wasm] Catch attempt to export non-existent memory

R=titzer@chromium.org
BUG=v8:5840

Review-Url: https://codereview.chromium.org/2633153002
Cr-Commit-Position: refs/heads/master@{#42384}

src/wasm/module-decoder.cc

@@ -457,7 +457,11 @@ class ModuleDecoder : public Decoder {
           }
           case kExternalMemory: {
             uint32_t index = consume_u32v("memory index");
-            if (index != 0) error("invalid memory index != 0");
+            // TODO(titzer): This should become more regular
+            // once we support multiple memories.
+            if (!module->has_memory || index != 0) {
+              error("invalid memory index != 0");
+            }
             module->mem_export = true;
             break;
           }

test/mjsunit/wasm/instantiate-module-basic.js

@@ -241,3 +241,9 @@ assertFalse(WebAssembly.validate(bytes(88, 88, 88, 88, 88, 88, 88, 88)));
 
   assertThrows(() => new WebAssembly.Instance(module, {"":{memory:memory}}), WebAssembly.LinkError);
 })();
+
+(function TestNoMemoryToExport() {
+  let builder = new WasmModuleBuilder();
+  builder.exportMemoryAs('memory');
+  assertThrows(() => builder.instantiate(), WebAssembly.CompileError);
+})();