gnrc_ipv6_nib: add address from netif to address validation timer

The `addr` parameter of the NIB's `_handle_dad()` function can come
from anywhere (e.g. in the fallback to classic SLAAC the destination
address of the IP header is used), so putting that pointer in a timer
is not a good idea. Instead we use the version of the address that is
stored within the interface.

sys/net/gnrc/network_layer/ipv6/nib/_nib-slaac.c

@@ -178,7 +178,8 @@ void _handle_dad(const ipv6_addr_t *addr)
     if (idx >= 0) {
         ipv6_addr_set_solicited_nodes(&sol_nodes, addr);
         _snd_ns(addr, netif, &ipv6_addr_unspecified, &sol_nodes);
-        _evtimer_add((void *)addr, GNRC_IPV6_NIB_VALID_ADDR,
+        _evtimer_add((void *)&netif->ipv6.addrs[idx],
+                     GNRC_IPV6_NIB_VALID_ADDR,
                      &netif->ipv6.addrs_timers[idx],
                      netif->ipv6.retrans_time);
     }