Get rid of static "zero" coefficient in coefficient.c

[dddejan]

Leads to leak reports when using as a library

static coefficient_t zero;
static int zero_initialized = 0;

static const coefficient_t* get_zero() {
  if (!zero_initialized) {
    zero_initialized = 1;
    zero.type = COEFFICIENT_NUMERIC;
    integer_construct(&zero.value.num);
  }
  return &zero;
}