libweston: Skip views without a parent

This prevents to trigger an assert within
weston_view_set_rel_position(), introduced with commit 'libweston: Split
weston_view_set_position() into rel and abs variants', which is hit when
a subsurface attempts to commit without having a parent surface set.

Fixes: #730

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reported-by: Colin Kinloch <collin.kinloch@collabora.com>