buffer: properly apply offset on fast path

PR-URL: #54391

src/node_buffer.cc

@@ -1481,7 +1481,7 @@ uint32_t FastWriteString(Local<Value> receiver,
 
   max_length = std::min<uint32_t>(dst.length() - offset, max_length);
 
-  memcpy(dst_data, src.data, max_length);
+  memcpy(dst_data + offset, src.data, max_length);
 
   return max_length;
 }