[wasm] Support a two-level namespace for imports.

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/macro-assembler.h"
 #include "src/objects.h"
 #include "src/v8.h"
 
 #include "src/wasm/decoder.h"
 #include "src/wasm/module-decoder.h"
@@ skipping 233 matching lines @@
             WasmImport* import = &module->import_table.back();
 
             const byte* sigpos = pc_;
             import->sig_index = consume_u16("signature index");
 
             if (import->sig_index >= module->signatures.size()) {
               error(sigpos, "invalid signature index");
             } else {
               import->sig = module->signatures[import->sig_index];
             }
+            const byte* pos = pc_;
             import->module_name_offset = consume_string("import module name");
+            if (import->module_name_offset == 0) {
+              error(pos, "import module name cannot be NULL");
+            }
             import->function_name_offset =
                 consume_string("import function name");
           }
           break;
         }
         case kDeclExportTable: {
           // Declares an export table.
           CheckForPreviousSection(sections, kDeclFunctions, true);
           int length;
           uint32_t export_table_count =
@@ skipping 366 matching lines @@
   if (function_start > function_end) return FunctionError("start > end");
   if (size > kMaxFunctionSize)
     return FunctionError("size > maximum function size");
   WasmFunction* function = new WasmFunction();
   ModuleDecoder decoder(zone, function_start, function_end, kWasmOrigin);
   return decoder.DecodeSingleFunction(module_env, function);
 }
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8