client: Don't inappropriatly close fds for zombie objects

commit 239ba39331420f953de35c337ae57db35573f9cb which was intended
to stop leaking fds in events for zombie objects didn't notice that
passing 0 to wl_connection_close_fds_in() would still close fds.

Test the fd count before calling.

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