Fix crash reported on list when renaming a child buffer and

Fix crash reported on list when renaming a child buffer and
    then trying to compile.

    There are really two problems here. One is that the renamed
    buffer needs to be reloaded. All kinds of files, etc, may not
    exist any more, if we've been saved to a new directory; our
    children e.g. may not be in the right place. And, in this case,
    we may no longer be a child of our old parent. Reloading will
    fix all of that.

    On reload, though, we need to clear our parent, since we may
    not have one any more. It will get reset if need be.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@40609 a592a061-630c-0410-9148-cb99ea01b6c8
(cherry picked from commit 9283cbdbcb3b33da28d2ecca6cd5665f39c7b388)