dbus-spawn: don't leave bad file descriptors being watched

dbus-spawn: don't leave bad file descriptors being watched

The code called from handle_watch() might close either or both of the
sockets we're watching, without cleaning up the DBusWatch. This results
in invalid file descriptors being passed to _dbus_poll(), which could
end up busy-looping on a POLLNVAL condition until the babysitter loses
its last ref (which automatically clears up both watches).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=32992
Bug-NB: NB#200248
Reviewed-by: Colin Walters <walters@verbum.org>