Refactor: pacemakerd: Remove more nesting in child_liveness()

Now, rc != pcmk_rc_ipc_unresponsive is equivalent to rc == pcmk_rc_ok at
the end of the changed section. Also, ipc_pid != 0 is basically
equivalent to rc == pcmk_rc_ok: pcmk__ipc_is_authentic_process_active()
returns pcmk_rc_ok only if it found a PID.

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