[wasm] Catch attempt to export non-existent memory

src/wasm/module-decoder.cc

 // Copyright 2015 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.
 
 #include "src/wasm/module-decoder.h"
 
 #include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/flags.h"
 #include "src/macro-assembler.h"
@@ skipping 439 matching lines @@
             break;
           }
           case kExternalTable: {
             WasmIndirectFunctionTable* table = nullptr;
             exp->index = consume_table_index(module, &table);
             if (table) table->exported = true;
             break;
           }
           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;
           }
           case kExternalGlobal: {
             WasmGlobal* global = nullptr;
             exp->index = consume_global_index(module, &global);
             if (global) {
               if (global->mutability) {
                 error("mutable globals cannot be exported");
               }
@@ skipping 764 matching lines @@
     table.push_back(std::move(func_asm_offsets));
   }
   if (decoder.more()) decoder.error("unexpected additional bytes");
 
   return decoder.toResult(std::move(table));
 }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8