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

src/wasm/module-decoder.cc

Index: src/wasm/module-decoder.cc
diff --git a/src/wasm/module-decoder.cc b/src/wasm/module-decoder.cc
index 401fb1823973049a9f616662a79b9699e91c8530..f81b8f440b410219a523c64ee2245d5cac7bb8e3 100644
--- a/src/wasm/module-decoder.cc
+++ b/src/wasm/module-decoder.cc
@@ -251,7 +251,11 @@ class ModuleDecoder : public Decoder {
             } 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");
           }