Implement RFC 6221 para 4.

If the link address in a relayed DHCPv6 message is zero, don't
use it for addresss selection.

src/rfc3315.c

@@ -245,6 +245,10 @@ static int dhcp6_maybe_relay(struct state *state, unsigned char *inbuff, size_t
 	      struct in6_addr align;
 	      /* the packet data is unaligned, copy to aligned storage */
 	      memcpy(&align, inbuff + 2, IN6ADDRSZ); 
+
+
+	      /* RFC6221 para 4 */
+	      if (!IN6_IS_ADDR_UNSPECIFIED(&align))
 		state->link_address = &align;
 	      /* zero is_unicast since that is now known to refer to the 
 		 relayed packet, not the original sent by the client */