MDEV-34397: "delete si" rather than "my_free(si)" in THD::register_slave()

In the error case of THD::register_slave(), there is undefined
behavior of Slave_info si because it is allocated via malloc()
(my_malloc), and cleaned up via delete().

This patch makes these consistent by switching si's cleanup
to use my_free.

sql/repl_failsafe.cc

@@ -159,7 +159,7 @@ int THD::register_slave(uchar *packet, size_t packet_length)
   return 0;
 
 err:
-  delete si;
+  my_free(si);
   my_message(ER_UNKNOWN_ERROR, errmsg, MYF(0)); /* purecov: inspected */
   return 1;
 }