_dbus_server_new_for_socket: Properly disconnect during error unwinding

_dbus_server_new_for_socket: Properly disconnect during error unwinding

_dbus_server_finalize_base() asserts that the socket has been
disconnected, but in some OOM code paths we would call it without
officially disconnecting. Do so.

This means we need to be a bit more careful about what is
socket_disconnect()'s responsibility to clean up, what is
_dbus_server_new_for_socket()'s responsibility, and what is the caller's
responsibility.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=89104