Fix cluster-fuzz regression with Workers when serializing empty string

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 333f7e440274b2334ace286e7be5e46064fa3959..4133283f1c4c6e7695cda898dc8c408eb0c82590 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -1538,8 +1538,10 @@ void SerializationData::WriteTag(SerializationTag tag) { data.Add(tag); }
 
 
 void SerializationData::WriteMemory(const void* p, int length) {
-  i::Vector<uint8_t> block = data.AddBlock(0, length);
-  memcpy(&block[0], p, length);
+  if (length > 0) {
+    i::Vector<uint8_t> block = data.AddBlock(0, length);
+    memcpy(&block[0], p, length);
+  }
 }