message: [zerocoin] Free memory from ToString()
  Simple fix for a minor memory leak
diff: @@ -196,6 +196,11 @@ std::string CBigNum::ToString(int nBase) const
  {
  char* c_str = mpz_get_str(NULL, nBase, bn);
  std::string str(c_str);
  + // Free c_str with the right free function:
  + void (*freefunc)(void *, size_t);
  + mp_get_memory_functions (NULL, NULL, &freefunc);
  + freefunc(c_str, strlen(c_str) + 1);
  +
  return str;
  }
  