networkd: fix ndisc assert 12452

allow ndisc client to configure on later stage.

The conf
```
[Match]
Name=enp7s0f0

[Network]
DHCP=true
IPv6PrivacyExtensions=true
IPv6AcceptRA=true <============================================
IPv6MTUBytes=1492
DNSDefaultRoute=false

[DHCP]
UseDNS=false
UseHostname=false
UseNTP=false
UseDomains=route
UseMTU=true

[IPv6AcceptRA]
UseDNS=no
UseDomains=route
```

The log
```
enp7s0f0: Gained IPv6LL
Assertion 'link->ndisc' failed at ../systemd-stable/src/network/networkd-link.c:1829, function link_acquire_ipv6_conf(). Abor
```

From the conf it's crear that RA accepting is enables. But hmm
initially when networkd started the ipv6ll was not there. Later
on it gailed that and tried to start ndisc client . Hence crashed.
So allow ndisc client to configure on later stage.

Closes systemd#12452

src/network/networkd-link.c

@@ -1825,7 +1825,11 @@ static int link_acquire_ipv6_conf(Link *link) {
         assert(link);
 
         if (link_ipv6_accept_ra_enabled(link)) {
-                assert(link->ndisc);
+                if (!link->ndisc) {
+                        r = ndisc_configure(link);
+                        if (r < 0)
+                                return r;
+                }
 
                 log_link_debug(link, "Discovering IPv6 routers");