networkd: add an assert

This error should never happen, so replace the check with an assert. The check
was anyway broken due to an uninitialized return value.

Reported by Thomas Hindoe Paaboel Andersen <phomes@gmail.com>.

src/network/networkd-link.c

@@ -1518,8 +1518,7 @@ int link_save(Link *link) {
         assert(link->state_file);
 
         state = link_state_to_string(link->state);
-        if (!state)
-                goto finish;
+        assert(state);
 
         r = fopen_temporary(link->state_file, &f, &temp_path);
         if (r < 0)