stream.rtmp: Fix is_usuable().
stream.rtmp: Fix is_usuable().

gtmanfred/livestreamer,intact/livestreamer,sbstp/streamlink,ethanhlc/streamlink,hmit/livestreamer,Masaz-/livestreamer,mmetak/streamlink,chrippa/livestreamer,charmander/livestreamer,melmorabity/streamlink,wolftankk/livestreamer,flijloku/livestreamer,lyhiving/livestreamer,streamlink/streamlink,caorong/livestreamer,gravyboat/streamlink,chhe/livestreamer,okaywit/livestreamer,back-to/streamlink,derrod/livestreamer,bastimeyer/streamlink,sbstp/streamlink,wlerin/streamlink,jtsymon/livestreamer,ethanhlc/streamlink,back-to/streamlink,melmorabity/streamlink,fishscene/streamlink,intact/livestreamer,blxd/livestreamer,chhe/livestreamer,javiercantero/streamlink,jtsymon/livestreamer,Feverqwe/livestreamer,hmit/livestreamer,Dobatymo/livestreamer,javiercantero/streamlink,chhe/streamlink,Saturn/livestreamer,bastimeyer/streamlink,programming086/livestreamer,streamlink/streamlink,chhe/streamlink,gtmanfred/livestreamer,Klaudit/livestreamer,Klaudit/livestreamer,chrippa/livestreamer,derrod/livestreamer,mmetak/streamlink,caorong/livestreamer,okaywit/livestreamer,wlerin/streamlink,flijloku/livestreamer,fishscene/streamlink,gravyboat/streamlink,Saturn/livestreamer,Feverqwe/livestreamer,programming086/livestreamer,Masaz-/livestreamer,blxd/livestreamer,beardypig/streamlink,wolftankk/livestreamer,beardypig/streamlink,charmander/livestreamer,Dobatymo/livestreamer,lyhiving/livestreamer