get next offset in a structure
id - structure type ID
offset - current offset
returns: -1 if bad structure type ID is passed
or no (more) offsets in the structure
otherwise returns next offset in a structure.
NOTE: IDA allows 'holes' between members of a
structure. It treats these 'holes'
as unnamed arrays of bytes.
This function returns a member offset or a hole offset.
It will return size of the structure if input
'offset' belongs to the last member of the structure.
NOTE: Union members are, in IDA's internals, located
at subsequent byte offsets: member 0 -> offset 0x0,
member 1 -> offset 0x1, etc...
long get_next_offset(long id, long offset);