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
(cherry picked from commit 9f939335)
(cherry picked from commit 743b771c)
(cherry picked from commit 1e20ca63)
(cherry picked from commit 655ae23e)