Handle wrong computation of quantity

  when parsing brainfuck code

  Previous commit introduced an assertion that can
  actually fail. In the hannoi example, the assertion failed.
  Luckily this was no malicious code that exploited the wrong
  computation in pre-assertion times.

  Simple reproduction of erroneous parsing:
  +--+  // does nothing but qunatity goes to zero with second -
  +---  // negative quantity. possible memory corruption exploit