networkd: add assert

It should not be possible to have a DHCP lease on a link without also having
an associated network. Add assert() to avoid compiler warnings.

Reported by Thomas H. P. Andersen

src/network/networkd-link.c

@@ -2152,6 +2152,8 @@ int link_save(Link *link) {
         }
 
         if (link->dhcp_lease) {
+                assert(link->network);
+
                 r = dhcp_lease_save(link->dhcp_lease, link->lease_file);
                 if (r < 0)
                         goto finish;