Handle AuthSwitchRequest responses to COM_CHANGE_USER

The COM_CHANGE_USER that is sent as a part of the reset process for a
persistent connection did not expect a AuthSwitchRequest packet to be sent
as that implies that the server did not take the authentication fast
path. In this case, an error message needs to be logged stating that the
server requested a different authentication plugin than was expected.

server/modules/protocol/MySQL/MySQLBackend/mysql_backend.c

@@ -774,6 +774,12 @@ gw_read_and_write(DCB *dcb)
                  * close the DCB and send an error to the client. */
                 log_error_response(dcb, reply);
             }
+            else if (result == MYSQL_REPLY_AUTHSWITCHREQUEST &&
+                     gwbuf_length(reply) > MYSQL_EOF_PACKET_LEN)
+            {
+                MXS_ERROR("Received AuthSwitchRequest to '%s' when '%s' was expected",
+                          (char*)GWBUF_DATA(reply) + 5, DEFAULT_MYSQL_AUTH_PLUGIN);
+            }
             else
             {
                 /** This should never happen */