MINOR: proxy: always properly reset the just freed default instance pointers

MINOR: proxy: always properly reset the just freed default instance pointers

In proxy_free_defaults(); none of the free() calls was followed by a
pointer reset. Not only it's hard to figure if one of them is duplicated,
but this code started to call other functions which might or might not
rely on such just freed pointers. Let's reset them as they should be to
make sure there will never be any case of use-after-free. The 3 functions
called there were inspected and are all unaffected by this so this remains
safe to do right now.