Initialize roots_output to fix -Wmaybe-uninitialized

With -O2:
../rootston/desktop.c: In function ‘desktop_surface_at’:
../rootston/desktop.c:714:18: error: ‘roots_output’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   if ((surface = layer_surface_at(roots_output,
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      &roots_output->layers[ZWLR_LAYER_SHELL_V1_LAYER_BOTTOM],
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      ox, oy, sx, sy))) {
      ~~~~~~~~~~~~~~~

rootston/desktop.c

@@ -682,7 +682,7 @@ struct wlr_surface *desktop_surface_at(struct roots_desktop *desktop,
 	struct wlr_surface *surface = NULL;
 	struct wlr_output *wlr_output =
 		wlr_output_layout_output_at(desktop->layout, lx, ly);
-	struct roots_output *roots_output;
+	struct roots_output *roots_output = NULL;
 	double ox = lx, oy = ly;
 	*view = NULL;