get member id
id - structure type ID
member_offset - member offset. The offset can be
any offset in the member. For example,
is a member is 4 bytes long and starts
at offset 2, then 2, 3, 4, 5 denote
the same structure member.
returns: -1 if bad structure type ID is passed or there is
no member at the specified offset.
otherwise returns the member id.
long get_member_id(long id, long member_offset);