desktop-shell: don't crash input-panel if no kbd focus

If a keyboard exists but it has no current focus, yet something asks the
input-panel to come up, we would crash here. Check that there is a focus
before attempting to use it.

Maybe there should not even exist a case where input-panel tries to come
up without a keyboard focus, but I am not sure there is no race where it
could happen.

In any case, this fix was brought up by the ivi-shell work, where I
suppose you can somehow hit it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Tanibata, Nobuhiko <ntanibata@jp.adit-jv.com>

desktop-shell/input-panel.c

@@ -66,7 +66,7 @@ show_input_panel_surface(struct input_panel_surface *ipsurf)
 	float x, y;
 
 	wl_list_for_each(seat, &shell->compositor->seat_list, link) {
-		if (!seat->keyboard)
+		if (!seat->keyboard || !seat->keyboard->focus)
 			continue;
 		focus = weston_surface_get_main_surface(seat->keyboard->focus);
 		ipsurf->output = focus->output;