Fix parsing empty IA_NA, IA_PD and invalid IA_ADDR options

Hi Steven,

some stateless DHCPv6 server could send ADVERT/REPLY with IA_NA/IA_PD without any inner suboptions.
odhcp6c treats such packets as invalid and doesn't do futher parsing to fallback to stateless mode or something else. with any garbage  if does - odhcp6 crashes.
patch fixes options length checking while packet validation and replies parsing.