properly destroying ffmpeg

[Roki100]

Issue:

im getting Cannot call write after a stream was destroyed in discord.js after doing <prism ffmpeg>.destroy() on dispatcher finish event

Steps to reproduce:

use prism ffmpeg and pass it into dispatcher, then destroy this ffmpeg on disaptcher finish (without it, ffmpegs does not despawn)

Further details:

• Operating System: windows 10 home
• Node.js version: 12.4.1
• Commit I'm using: prism 1.2.0 (that one that comes with discord.js master)

My reason of creating this issue

i cant seem to destroy ffmpeg without errors so my question is how can i destroy it properly
// it happens only while piping ytdl to transcoder, using -i url in ffmpeg doesnt throw that