resolved: fix loop on packets with pseudo dns types

Reported by Karim Hossen & Thomas Imbert from Sogeti ESEC R&D.
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1725351



Change-Id: I858cbf85a172c2565cfdb0ccb96484e4db09068b
Signed-off-by: Lei wang <leiwan@codeaurora.org>