Fix: libcrmcommon: Avoid use-after-free in mark_xml_changes()

mark_xml_changes() with check_top=true may free new_child via
pcmkapply_creation_acl(). Calling pcmkxml_next(new_child) after that
is a use-after-free error. This caused a seg fault on FreeBSD.

Regression introduced in 12710e69.

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