Fix inifinte loop in esp32 RIO handling. (#12736)

If the router is advertising a default prefix (::/0), the route
handler is going into an infinite loop.

Route information options can only have one prefix anyway, so the
loop here is overkill and likely to introduce bad routes. Changing
this to a simple check.

Test: Configured radvd to advertise a default route, saw infinite
      loop on upstream, no problems with this code. Regular routes
      are being processed correctly.

examples/platform/esp32/route_hook/esp_route_hook.c

@@ -94,7 +94,7 @@ static void ra_recv_handler(struct netif * netif, const uint8_t * icmp_payload,
             uint8_t rio_data_len     = opt_len - sizeof(rio_header_t);
 
             ESP_LOGI(TAG, "Received RIO");
-            for (; rio_data_len >= prefix_len_bytes; rio_data_len -= prefix_len_bytes, rio_data += prefix_len_bytes)
+            if (rio_data_len >= prefix_len_bytes)
             {
                 ip6_addr_t prefix;
                 esp_route_entry_t route;