fix make buffer

ffmpeg.go

package main

import (
	"bytes"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
)

func check(err error) {
	if err != nil {
		log.Fatalln(err)
	}
}

func main() {
	file, err := os.Open("test.mp3") // open file
	check(err)
	
	defer file.Close()
	buf, err := ioutil.ReadAll(file)
	check(err)

	cmd := exec.Command("ffmpeg", "-y", // Yes to all
		//"-hide_banner", "-loglevel", "panic", // Hide all logs
		"-i", "pipe:0", // take stdin as input
		"-map_metadata", "-1", // strip out all (mostly) metadata
		"-c:a", "libmp3lame", // use mp3 lame codec
		"-vsync", "2", // suppress "Frame rate very high for a muxer not efficiently supporting it"
		"-b:a", "128k", // Down sample audio birate to 128k
		"-f", "mp3", // using mp3 muxer (IMPORTANT, output data to pipe require manual muxer selecting)
		"pipe:1", // output to stdout
	)
	
        // resultBuffer := bytes.NewBuffer(make([]byte, 0)) // uncomment this line if you don't want to preallocate the buffer
	resultBuffer := bytes.NewBuffer(make([]byte, 5*1024*1024)) // pre allocate 5MiB buffer

	cmd.Stderr = os.Stderr // bind log stream to stderr
	cmd.Stdout = resultBuffer // stdout result will be written here

	stdin, err := cmd.StdinPipe() // Open stdin pipe
	check(err)

	err = cmd.Start() // Start a process on another goroutine
	check(err)

	_, err = stdin.Write(buf) // pump audio data to stdin pipe
	check(err)

	err = stdin.Close() // close the stdin, or ffmpeg will wait forever
	check(err)

	err = cmd.Wait() // wait until ffmpeg finish
	check(err)

	outputFile, err := os.Create("out.mp3") // create new file
	check(err)
	defer outputFile.Close()
	
	_, err = outputFile.Write(resultBuffer.Bytes()) // write result buffer to file
	check(err)
}