initialize the filler byte to 0

```
COM_STMT_PREPARE OK
  OK response to a COM_STMT_PREPARE packet

  direction: server -> client

  payload:
    1              [00] OK
    4              statement-id
    2              num-columns
    2              num-params
    1              [00] filler
    2              warning count
```

According to the protocol 10th byte (filler) of the response should be
0. introduced by 02432b2b27.

lib/MySQL_Protocol.cpp

@@ -704,6 +704,7 @@ bool MySQL_Protocol::generate_STMT_PREPARE_RESPONSE(uint8_t sequence_id, MySQL_S
 	hdr.pkt_length=12;
 	memcpy(okpack,&hdr,sizeof(mysql_hdr)); // copy header
 	okpack[4]=0;
+	okpack[13]=0;
 	okpack[15]=0;
 	if (_stmt_id) {
 		memcpy(okpack+5,&_stmt_id,sizeof(uint32_t));