itoa_left large values

[asimes]

The following does not pass the 3rd iteration:

BOOST_AUTO_TEST_CASE(iota_left) {
    char bufA[32];
    char bufB[32];
    for (unsigned long long i = 9223372036854775807ull; i <= 9223372036854775809ull; ++i) {
        snprintf(bufA, 21, "%llu" , i);
        utxx::itoa_left(bufB, i);
        BOOST_REQUIRE(std::strcmp(bufA, bufB) == 0);
    }
}