Saturn: non-incremental mode crashes with `p is not defined`

cc @jakeread

Non-incremental mode seems to run in to a reference error:

puts new position for 31
VM53 saturn.js:490 lookahead
bootstrap.js:52 ERR @ top of loop: ReferenceError: p is not defined
    at rampPass (VM53 saturn.js:268)
    at Saturn.loop (VM53 saturn.js:515)
    at Manager.loop (manager.js:705)
    at bootloop (bootstrap.js:50)
bootloop @ bootstrap.js:52
bootstrap.js:55 loop halting, mgr bailing
bootloop @ bootstrap.js:55

System in question (which we modify by lowering velocity and disabling incremental mode, also doesn't work with simple one-motor example):

https://gitlab.cba.mit.edu/classes/863.19/CBA/students/cuttlefish-iLathe/blob/master/save/systems/applejack-XA-YD-ZC.json