get offset of the last member of a structure
id - structure type ID
returns: -1 if bad structure type ID is passed
or structure has no members
otherwise returns offset of the last member.
NOTE: IDA allows 'holes' between members of a
structure. It treats these 'holes'
as unnamed arrays of bytes.
NOTE: Union members are, in IDA's internals, located
at subsequent byte offsets: member 0 -> offset 0x0,
member 1 -> offset 0x1, etc...
long get_last_member(long id);