dhcp: Properly handle unsuccessful cases for DHCP Ack

Ignore DHCP Ack packets if they are found to be erroneous or have
an unexpected xid by setting the result to zero instead of propagating
an error. Also remember to report a DHCP Nak and stop the DHCP client.

src/libsystemd-dhcp/dhcp-client.c

@@ -897,6 +897,9 @@ static int client_receive_raw_message(sd_event_source *s, int fd,
                         client->receive_message =
                                 sd_event_source_unref(client->receive_message);
                 }
+
+                r = 0;
+
                 break;
 
         case DHCP_STATE_INIT:
@@ -910,7 +913,7 @@ static int client_receive_raw_message(sd_event_source *s, int fd,
         }
 
 error:
-        if (r < 0)
+        if (r < 0 || r == DHCP_EVENT_NO_LEASE)
                 return client_stop(client, r);
 
         return 0;