ifblock: fix clash with specials when freeing objects

ifblock: fix clash with specials when freeing objects

Murphy hit me again: in my naive attempt to fix the clash between
ifblocks and objects parsing text objects due to the double use of the
'sub' field, I overlooked this problem with reusing the 'special_data'
field. So here comes the real thing (TM), donating ifblocks their own
field for pointing to the jump target.