Fix out-of-bounds write during voting with duplicate ed25519 keys

In dirserv_compute_performance_thresholds, we allocate arrays based
on the length of 'routers', a list of routerinfo_t, but loop over
the nodelist. The 'routers' list may be shorter when relays were
filtered by routers_make_ed_keys_unique, leading to an out-of-bounds
write on directory authorities.

This bug was originally introduced in 26e89742, but it doesn't look
possible to trigger until routers_make_ed_keys_unique was introduced
in 13a31e72.

Fixes bug 19032; bugfix on tor 0.2.8.2-alpha.

src/or/dirserv.h

@@ -50,7 +50,7 @@ int list_server_status_v1(smartlist_t *routers, char **router_status_out,
 int dirserv_dump_directory_to_string(char **dir_out,
                                      crypto_pk_t *private_key);
 char *dirserv_get_flag_thresholds_line(void);
-void dirserv_compute_bridge_flag_thresholds(const smartlist_t *routers);
+void dirserv_compute_bridge_flag_thresholds(void);
 
 int directory_fetches_from_authorities(const or_options_t *options);
 int directory_fetches_dir_info_early(const or_options_t *options);