comp-wayland: use safe foreach when destroying outputs

wl_list_for_each dereference's output to increment the
next iteration of the loop.  However, output is free'd
inside the loop resulting in a dereference to free'd
memory.

Use wl_list_for_each_safe instead, which is designed to
handle this kind of pattern.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor-wayland.c

@@ -1997,9 +1997,9 @@ err_free:
 static void
 wayland_compositor_destroy(struct wayland_compositor *c)
 {
-	struct weston_output *output;
+	struct weston_output *output, *next;
 
-	wl_list_for_each(output, &c->base.output_list, link)
+	wl_list_for_each_safe(output, next, &c->base.output_list, link)
 		wayland_output_destroy(output);
 
 	c->base.renderer->destroy(&c->base);