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 scanMode(void) {
    redisReply *reply;
    unsigned long long cur = 0;

    do {
        if (config.pattern)
            reply = redisCommand(context,"SCAN %llu MATCH %s",
                cur,config.pattern);
        else
            reply = redisCommand(context,"SCAN %llu",cur);
        if (reply == NULL) {
            printf("I/O error\n");
            exit(1);
        } else if (reply->type == REDIS_REPLY_ERROR) {
            printf("ERROR: %s\n", reply->str);
            exit(1);
        } else {
            unsigned int j;

            cur = strtoull(reply->element[0]->str,NULL,10);
            for (j = 0; j < reply->element[1]->elements; j++)
                printf("%s\n", reply->element[1]->element[j]->str);
        }
        freeReplyObject(reply);
    } while(cur != 0);

    exit(0);
}

static void scanMode(void) {
    redisReply *reply;
    unsigned long long cur = 0;

    do {
        if (config.pattern)
            reply = redisCommand(context,"SCAN %llu MATCH %s",
                cur,config.pattern);
        else
            reply = redisCommand(context,"SCAN %llu",cur);
        if (reply == NULL) {
            printf("I/O error\n");
            exit(1);
        } else if (reply->type == REDIS_REPLY_ERROR) {
            printf("ERROR: %s\n", reply->str);
            exit(1);
        } else {
            unsigned int j;

            cur = strtoull(reply->element[0]->str,NULL,10);
            for (j = 0; j < reply->element[1]->elements; j++)
                printf("%s\n", reply->element[1]->element[j]->str);
        }
        freeReplyObject(reply);
    } while(cur != 0);

    exit(0);
}
