libweston: fix crash when a client queries a destroyed output

A client can call get_xdg_output() at any time. If the output was destroyed,
then all output resources are orphaned. This includes clearing the user data of
the resource.
This can happen, for example, when a monitor or HDMI switch quickly toggles the
HDMI hotplug pin.

So check if a head is associated with the resource. If not, just create the
resource and then return immediately.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>