clutter/actor-destroy: Check that destroying the actor clears the children

Commit df7d8e2c highlights a crash on test_destroy_destroy, in fact it could
happen that calling clutter_actor_destroy on a child while iterating on the
list, would implicitly call test_destroy_remove that tries to modify the list
at the same time. Causing a memory error.

So instead of manually free the children list, just ensure that this list is
valid and that when the object destruction is done, this is free'd.

See: GNOME/mutter!576

GNOME/mutter!577
4 jobs for !577 with test-actor-destroy-fix in 15 minutes and 23 seconds
latest detached
Status Job ID Name Coverage
  Review
passed #301288
check-commit-log

00:00:26

 
  Build
passed #301289
build-mutter

00:10:31

 
  Test
passed #301291
can-build-gnome-shell

00:01:25

failed #301290
test-mutter

00:04:25

 
Name Stage Failure
failed
test-mutter Test

Ok: 82
Expected Fail: 0
Fail: 6
Unexpected Pass: 0
Skipped: 0
Timeout: 0

ERROR: Job failed: exit code 1