plugin.c: Advance the llentry_t pointer before calling the shutdown callback.

plugin.c: Advance the llentry_t pointer before calling the shutdown callback.

> Important note: Unregistering the shutdown callback during shutdown is
> somewhat tricky. Doing that frees the llist_t entry in plugin.c:list_shutdown.
> This might cause a segfault in plugin_shutdown_all () when it tries to access
> the next pointer of that entry. If plugin_unregister_shutdown () is the last
> statement before the return from the shutdown callback this should be save in
> about 99% of all cases. Still there should be some better way to handle this.

This is an imho nice way to fix it.

Signed-off-by: Sebastian Harl <sh@tokkee.org>