[xwl] Fix xcb to wayland clipboard

A seemingly unrelated change to cursors broke xwayland's clipboard syncing, presumably because of a change in how startup is ordered with regards to syncing between our internal wayland connection causing a crash.

It was fixed with a flush, but ultimately guarding things not being initalised works better.

Long term plans are to finish removing the internal connection.

See xwayland_selections_test in autotests which reproduces this issue.