net: route: Do not try to access null link layer address

In some cases the lladdr might not be set, currently this is
seen with RPL unit tests, in which case we must not access
the lladdr.

Jira: ZEP-2330

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

subsys/net/ip/route.c

@@ -739,6 +739,11 @@ int net_route_packet(struct net_pkt *pkt, struct in6_addr *nexthop)
 		return -ESRCH;
 	}
 
+	if (!net_pkt_ll_src(pkt)->addr) {
+		NET_DBG("Link layer source address not set");
+		return -EINVAL;
+	}
+
 	/* Sanitycheck: If src and dst ll addresses are going to be same,
 	 * then something went wrong in route lookup.
 	 */