Make sure forked process has stopped when deleting MythSystem

This forces a termination in a forked MythSystem process when the
MythSystem object is destroyed, preventing a situation where the private
internal object tries to access resources from the no-longer-existent
parent object.

Fixes #11412
(cherry picked from commit 9c51ad39a65eee7a3f985a127700fbc8473ffb4e)