desktop-shell: Do not try to add a child to a non-existent parent

Introduced with a8da2084, it seems that there are cases when there's no parent available (zenity, for instance).

Removes any potential child and re-initialize it, in case the parent is not set. (Simon Ser)

Fixes: #340 (closed)

Signed-off-by: Marius Vlad marius.vlad@collabora.com Reviewed-by: Simon Ser contact@emersion.fr Reported-by: n3rdopolis bluescreenavenger@gmail.com