net: consider option-length when parsing NIS domain

When parsing option 40 (network information service domain) the
option length is in variable 'oplen' and not in 'size'.

Addresses-Coverity-ID: 492765 Uninitialized variables (UNINIT)
Fixes: 8ab388bf ("net: add support to parse the NIS domain for the dhcp options")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>