secchan: Fix segfault due to access-after-free in expiration.

secchan: Fix segfault due to access-after-free in expiration.

classifier_for_each() keeps a pointer to the *next* rule to be visited, so
that the rule currently be visited can be deleted.  That means that if
the callback frees the next rule to be visited, then we get an
access-after-free error.

In particular, this was occurring when expire_rule() expired a superflow
whose