main: let load_wayland_backend() clean up on error

main: let load_wayland_backend() clean up on error

weston_wayland_backend_config_release() does not bother reinitializing
the structure, it simply frees what is there. This leads to a structure
containing garbage, which might be a surprise.

Require, that if load_wayland_backend_config() fails, the caller must
call weston_wayland_backend_config_release() regardless.

Signed-off-by: Benoit Gschwind <gschwind@gnu-log.net>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
[Pekka: rewrote commit message]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>