fix: Cancel run promise if the exit() is called

[pikax]

The .run() promise will never be resolved if exit is called.

This PR will reject the run promise if is running:

const longRun = ffmpeg.run([/*args*/])

setTimeout(()=> {
  ffmpeg.exit()
}, timeout)

try{
  await longRun(); // before everything after this will never run
 } catch { 
  // handle error
 }
console.log('continue doing work')

[cainiaokan]

this PR needs to be merged ASAP. this bug really annoys me cause it breaks promise mechanism.

[rap2hpoutre]

Thank you @pikax

[jeromewu]

Thanks for the PR!