Guard against mutually recursive rules in a startloop...endloop block

If you happen to have a malformed loop containing mutually recursive rules, something like:

    startloop
      [ Player ] -> [ Crate ]
      [ Crate ] -> [ Player ]
    endloop

this can hang the browser tab. My fix just terminates these loops after 50 iterations, like how it works for self-recursive rules, though I'm unsure if 50 is too small a limit.