desktop-shell: Avoid retrieving output's width/height if none present

As in some circumstances there could be no output connected, avoid
retrieving the width/height of the output if none was found/connected.

Fixes: #384

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

desktop-shell/shell.c

@@ -2697,8 +2697,10 @@ set_fullscreen(struct shell_surface *shsurf, bool fullscreen,
 		shell_surface_set_output(shsurf, output);
 		shsurf->fullscreen_output = shsurf->output;
 
-		width = shsurf->output->width;
-		height = shsurf->output->height;
+		if (shsurf->output) {
+			width = shsurf->output->width;
+			height = shsurf->output->height;
+		}
 	} else if (weston_desktop_surface_get_maximized(desktop_surface)) {
 		get_maximized_size(shsurf, &width, &height);
 	}