fix double-free and off-by-one bugs in list cat
This addresses a double-free bug with the "clear" message to the right inlet. It also fixes an off-by-one error when sending a "selector_one, selector_two" message to the input. (Previously that input would double the second selector for a three-element list.)
Some other considerations:
- why "clear" to the 2nd inlet? To be consistent with list classes should that inlet take a "bang" message?
- would have been nice to only output on "bang" to the left inlet, but I guess that ship has sailed by now