get_prev_offset

get previous 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 previous 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' is bigger than the structure size.
         NOTE: Union members are, in IDA's internals, located
               at subsequent byte offsets: member 0 -> offset 0x0,
               member 1 -> offset 0x1, etc...

long get_prev_offset(long id, long offset);