[BUG] Memory leak in BigInt::getString() method.

[shude]

Hi, i found memory leak in BigInt::getString() method.
Here is a possible solution:

std::string BigInt::getString(int base) const {
    char *str = mpz_get_str(nullptr, base, data); // mpz_get_str  allocated memory - it must be free 
    std::string result = std::string(str);
    free(str);
    return result;
}