[ffmpegmux] Fixed bug of an invisible terminal
[ffmpegmux] Fixed bug of an invisible terminal

if a ffmpeg stream gets closed with ctrl + c the terminal will get
invisible,
this patch will fix it.

gravyboat/streamlink,streamlink/streamlink,melmorabity/streamlink,bastimeyer/streamlink,javiercantero/streamlink,chhe/streamlink,streamlink/streamlink,bastimeyer/streamlink,wlerin/streamlink,gravyboat/streamlink,wlerin/streamlink,back-to/streamlink,melmorabity/streamlink,beardypig/streamlink,chhe/streamlink,back-to/streamlink,beardypig/streamlink,javiercantero/streamlink