When workspace_wrap_children is called on a workspace which has a
fullscreen child and the fullscreen child is a direct child of the
workspace, sway would crash.
The workspace's fullscreen pointer is unset when the fullscreen
container is detached and applied again when added to a parent, but in
this case the parent hadn't yet been added to the workspace which meant
con->workspace was NULL.
The fix makes container_handle_fullscreen_reparent return if there's no
workspace, and the fullscreen pointer is reapplied in
workspace_wrap_children.
df95c61044