According to RFC 2460 the length field of the Hop-by-Hop options header
should use a 8-octet unit (multiple of 8 byte).
In a normal configuration the RPL_HOP_BY_HOP_LEN define is 8, so the
current implementation works, but if RPL_HOP_BY_HOP_LEN is a multiple
of 8 the length is not calculated correctly.