if we use mp_set_int() instead of mp_set(), then we can use
full unsigned long integers instead of single digits. This
seems a lot safer, especially for future versions of the
random algorithm.
pb_init() which is called by init_polynom_size() will call
mp_init_copy() on chara which means that chara being a pointer
itself is useless. We can clear it directly after it has been
copied.