wayland-client: Initialize newly created wl_proxys to zero

Up until now, newly created wl_proxys (with proxy_create or
wl_proxy_create_for_id) are not initialized properly after memory
allocation. The wl_display object in contrast is. To prevent giving
uninitialized data to the user (e.g. user_data) an appropriate memset
has been added. Also, after a memset members don't have to be
explicitly initialized with zero anymore.

Signed-off-by: Nils Chr. Brause <nilschrbrause@googlemail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>