Wrong hook, because what gets passed to wp is the
Current WordPress environment instance
not
The WP_Query instance
like it is e.g. passed to loop_start.
You could actually just do your check inside the function callback you hook to loop_start.
Edit/note:
You could just have done global $wp_query; inside the wp hook, it is actually the first one where access to $wp_query is possible, to have access to it – not that I recommend doing it though. A better place – or at least the one I probably would go for – to hook into is the pre_get_posts action, which does get passed $query – the WP_Query object – by reference.