sway: extensions: only unlock if lock_surfaces.length == 0
sway: extensions: only unlock if lock_surfaces.length == 0

When destroying lock surfaces, we really should only unlock a
desktop_shell if the set of lock surfaces has dropped to zero (since
callers need to do a set_lock_surface for every output).

Signed-off-by: Aleksa Sarai <b305c9b0212d6145eea67bbffd24e29cb79c759c@cyphar.com>

sleep-walker/sway,ascent12/sway,taiyu-len/sway,mikkeloscar/sway,johalun/sway,1ace/sway,taiyu-len/sway,mikkeloscar/sway,4e554c4c/sway,4e554c4c/sway,1ace/sway,ascent12/sway,ptMuta/sway,1ace/sway,ascent12/sway,SirCmpwn/sway,taiyu-len/sway,sleep-walker/sway