SnapshotCreator: start from existing snapshot if we have one

src/api.cc

 // Copyright 2012 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/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 493 matching lines @@
 
 SnapshotCreator::SnapshotCreator(intptr_t* external_references,
                                  StartupData* existing_snapshot) {
   i::Isolate* internal_isolate = new i::Isolate(true);
   Isolate* isolate = reinterpret_cast<Isolate*>(internal_isolate);
   SnapshotCreatorData* data = new SnapshotCreatorData(isolate);
   data->isolate_ = isolate;
   internal_isolate->set_array_buffer_allocator(&data->allocator_);
   internal_isolate->set_api_external_references(external_references);
   isolate->Enter();
-  if (existing_snapshot) {
-    internal_isolate->set_snapshot_blob(existing_snapshot);
+  const StartupData* blob = existing_snapshot
+                                ? existing_snapshot
+                                : i::Snapshot::DefaultSnapshotBlob();
+  if (blob && blob->raw_size > 0) {
+    internal_isolate->set_snapshot_blob(blob);
     i::Snapshot::Initialize(internal_isolate);
   } else {
     internal_isolate->Init(nullptr);
   }
   data_ = data;
 }
 
 SnapshotCreator::~SnapshotCreator() {
   SnapshotCreatorData* data = SnapshotCreatorData::cast(data_);
   DCHECK(data->created_);
@@ skipping 9731 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8