gnrc_sixlowpan_nd: fixed crash on gnrc_sixlowpan_nd_next_hop_l2addr.

When `gnrc_ndp_node_next_hop_l2addr` cannot resolve L2 address, it creates a
temporary neighbor cache entry with interface `KERNEL_PID_UNDEF` (unless the
interface is already known) to send a neighbor solicitation. When another packet
directed to the same address is going to sent before receiving a neighbor
advertisement, `gnrc_sixlowpan_nd_next_hop_l2addr` gets the temporary neighbor
cache entry and calls `gnrc_ipv6_netif_get` with `KERNEL_PID_UNDEF`, resulting
get a `NULL`. We must check `NULL` before dereference it.

FYI, both `gnrc_ndp_node_next_hop_l2addr` and
`gnrc_sixlowpan_nd_next_hop_l2addr` are enabled when
`gnrc_sixlowpan_border_router_default` module is enabled with `GNRC_NETIF_NUMOF`
is greater than 1:

gnrc_sixlowpan_border_router_default
→ gnrc_ipv6_router_default
→ gnrc_ndp_router (if GNRC_NETIF_NUMOF > 1)
→ gnrc_ndp_node
→ gnrc_ndp_node_next_hop_l2addr is called from _next_hop_l2addr

gnrc_sixlowpan_border_router_default
→ gnrc_sixlowpan_nd_border_router
→ gnrc_sixlowpan_nd_router
→ gnrc_sixlowpan_nd
→ gnrc_sixlowpan_nd_next_hop_l2addr is called from _next_hop_l2addr

sys/net/gnrc/network_layer/sixlowpan/nd/gnrc_sixlowpan_nd.c

@@ -154,7 +154,7 @@ kernel_pid_t gnrc_sixlowpan_nd_next_hop_l2addr(uint8_t *l2addr, uint8_t *l2addr_
     if (nc_entry != NULL) {
         gnrc_ipv6_netif_t *ipv6_if = gnrc_ipv6_netif_get(nc_entry->iface);
         /* and interface is not 6LoWPAN */
-        if (!(ipv6_if->flags & GNRC_IPV6_NETIF_FLAGS_SIXLOWPAN) ||
+        if (!(ipv6_if == NULL || ipv6_if->flags & GNRC_IPV6_NETIF_FLAGS_SIXLOWPAN) ||
                 /* or entry is registered */
                 (gnrc_ipv6_nc_get_type(nc_entry) == GNRC_IPV6_NC_TYPE_REGISTERED)) {
         next_hop = dst;