SnapshotCreator: start from existing snapshot if we have one

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 866a0bc22441e84f5339397ba42f9ce2c190ea44..483f07a9f241d1df59627af1c1d52c1d4cf19dc8 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -511,8 +511,11 @@ SnapshotCreator::SnapshotCreator(intptr_t* external_references,
   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);