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 latencyModePrint(long long min, long long max, double avg, long long count) {
    if (config.output == OUTPUT_STANDARD) {
        printf("min: %lld, max: %lld, avg: %.2f (%lld samples)",
                min, max, avg, count);
        fflush(stdout);
    } else if (config.output == OUTPUT_CSV) {
        printf("%lld,%lld,%.2f,%lld\n", min, max, avg, count);
    } else if (config.output == OUTPUT_RAW) {
        printf("%lld %lld %.2f %lld\n", min, max, avg, count);
    }
}

static void latencyModePrint(long long min, long long max, double avg, long long count) {
    if (config.output == OUTPUT_STANDARD) {
        printf("min: %lld, max: %lld, avg: %.2f (%lld samples)",
                min, max, avg, count);
        fflush(stdout);
    } else if (config.output == OUTPUT_CSV) {
        printf("%lld,%lld,%.2f,%lld\n", min, max, avg, count);
    } else if (config.output == OUTPUT_RAW) {
        printf("%lld %lld %.2f %lld\n", min, max, avg, count);
    }
}
