MXS-1846: Send correct packet number in errors

The mysql_create_standard_error function accepted a packet number as a
parameter but did not use it as the actual packet number. As the value it
used happened to coincide with 50% of the use-cases, it went unnoticed.

The remaining 50% occurred when a KILL command was executed with an
unknown connection ID.

server/modules/protocol/MySQL/mysql_common.cc

@@ -314,7 +314,7 @@ mysql_create_standard_error(int packet_number,
     gw_mysql_set_byte3(mysql_packet_header, mysql_payload_size);
 
     // write packet number, now is 0
-    mysql_packet_header[3] = 0;
+    mysql_packet_header[3] = packet_number;
     memcpy(outbuf, mysql_packet_header, sizeof(mysql_packet_header));
 
     // current buffer pointer