SnapshotCreator: start from existing snapshot if we have one

test/cctest/wasm/test-run-wasm-module.cc

Index: test/cctest/wasm/test-run-wasm-module.cc
diff --git a/test/cctest/wasm/test-run-wasm-module.cc b/test/cctest/wasm/test-run-wasm-module.cc
index 03ffb7ac65e5917db85ba5957147e552c4cc4927..a3dd35713dfb23ca4c54fdd2a978716de1b8e544 100644
--- a/test/cctest/wasm/test-run-wasm-module.cc
+++ b/test/cctest/wasm/test-run-wasm-module.cc
@@ -237,9 +237,10 @@ class WasmSerializationTest {
   }
 
   void InvalidateVersion() {
-    uint32_t* buffer = reinterpret_cast<uint32_t*>(
-        const_cast<uint8_t*>(serialized_bytes_.first));
-    buffer[SerializedCodeData::kVersionHashOffset] = Version::Hash() + 1;
+    uint32_t* slot = reinterpret_cast<uint32_t*>(
+        const_cast<uint8_t*>(serialized_bytes_.first) +
+        SerializedCodeData::kVersionHashOffset);
+    *slot = Version::Hash() + 1;
   }
 
   void InvalidateWireBytes() {
@@ -247,6 +248,13 @@ class WasmSerializationTest {
            wire_bytes_.second / 2);
   }
 
+  void InvalidateLength() {
+    uint32_t* slot = reinterpret_cast<uint32_t*>(
+        const_cast<uint8_t*>(serialized_bytes_.first) +
+        SerializedCodeData::kPayloadLengthOffset);
+    *slot = 0xfefefefeu;
+  }
+
   v8::MaybeLocal<v8::WasmCompiledModule> Deserialize() {
     ErrorThrower thrower(current_isolate(), "");
     v8::MaybeLocal<v8::WasmCompiledModule> deserialized =
@@ -406,6 +414,17 @@ TEST(DeserializeNoSerializedData) {
   Cleanup();
 }
 
+TEST(DeserializeInvalidLength) {
+  WasmSerializationTest test;
+  {
+    HandleScope scope(test.current_isolate());
+    test.InvalidateLength();
+    test.DeserializeAndRun();
+  }
+  Cleanup(test.current_isolate());
+  Cleanup();
+}
+
 TEST(DeserializeWireBytesAndSerializedDataInvalid) {
   WasmSerializationTest test;
   {