connection: close_fds() should only remove fds it closed from the buffer

All current callers close all fds, so this has gone unnoticed, but if
we close less than all fds with close_fds() we leak all the unclosed
ones and ruin further event demarshalling.

A future patch will close less than the full buffer's worth of fds,
so this is now noticed.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>