Fix: fencing: Don't set stonith action to pending if fork fails

Currently, we set a stonith action to pending if
services_action_async_fork_notify() returns true. However, "true" means
that the svc_action should not be freed. This might be because the
svc_action forked successfully and is pending, or it might be because
the svc_action has already been freed.

In the case of stonith actions, if we fail to fork, the stonith_action_t
object stored in svc_action->cb_data gets freed by the done callback,
and services_action_async_fork_notify() returns true. If we try to set
the action to pending, it causes a segfault.

This commit moves the "set to pending" step to the
stonith_action_async_forked() callback. We avoid the segfault and only
set it to pending if it's actually pending.

A slight difference in ordering was required to achieve this. Now, the
action gets set to pending immediately before being added to the
mainloop, instead of immediately after.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>