ipv6: fix dst/hop header option parsing

The extension header option parsing used a uint8_t internally. However
much bigger option sizes are valid.

Prscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/8
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/8