Security: fix redis-cli buffer overflow.

Thanks to Fakhri Zulkifli for reporting it.

The fix switched to dynamic allocation, copying the final prompt in the
static buffer only at the end.
static void intrinsicLatencyMode(void) {
    long long test_end, run_time, max_latency = 0, runs = 0;

    run_time = config.intrinsic_latency_duration*1000000;
    test_end = ustime() + run_time;
    signal(SIGINT, intrinsicLatencyModeStop);

    while(1) {
        long long start, end, latency;

        start = ustime();
        compute_something_fast();
        end = ustime();
        latency = end-start;
        runs++;
        if (latency <= 0) continue;

        /* Reporting */
        if (latency > max_latency) {
            max_latency = latency;
            printf("Max latency so far: %lld microseconds.\n", max_latency);
        }

        double avg_us = (double)run_time/runs;
        double avg_ns = avg_us * 1e3;
        if (force_cancel_loop || end > test_end) {
            printf("\n%lld total runs "
                "(avg latency: "
                "%.4f microseconds / %.2f nanoseconds per run).\n",
                runs, avg_us, avg_ns);
            printf("Worst run took %.0fx longer than the average latency.\n",
                max_latency / avg_us);
            exit(0);
        }
    }
}

static void intrinsicLatencyMode(void) {
    long long test_end, run_time, max_latency = 0, runs = 0;

    run_time = config.intrinsic_latency_duration*1000000;
    test_end = ustime() + run_time;
    signal(SIGINT, intrinsicLatencyModeStop);

    while(1) {
        long long start, end, latency;

        start = ustime();
        compute_something_fast();
        end = ustime();
        latency = end-start;
        runs++;
        if (latency <= 0) continue;

        /* Reporting */
        if (latency > max_latency) {
            max_latency = latency;
            printf("Max latency so far: %lld microseconds.\n", max_latency);
        }

        double avg_us = (double)run_time/runs;
        double avg_ns = avg_us * 1e3;
        if (force_cancel_loop || end > test_end) {
            printf("\n%lld total runs "
                "(avg latency: "
                "%.4f microseconds / %.2f nanoseconds per run).\n",
                runs, avg_us, avg_ns);
            printf("Worst run took %.0fx longer than the average latency.\n",
                max_latency / avg_us);
            exit(0);
        }
    }
}
