dns: improve response name parsing

Improve parsing of names with multiple pointers following each other.

Bug 1141. https://redmine.openinfosecfoundation.org/issues/1141

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/273
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/192