fix(streams): handle interface conversion panic in NewStream() at internal/streams

[skrashevich]

panic: interface conversion: interface {} is map[string]interface {}, not string

goroutine 1 [running]:
github.com/AlexxIT/go2rtc/internal/streams.NewStream({0x1010ef3e0?, 0x140001305b8?})
        /Users/svk/Documents/GitHub/go2rtc/internal/streams/stream.go:26 +0x3b0
github.com/AlexxIT/go2rtc/internal/streams.Init()
        /Users/svk/Documents/GitHub/go2rtc/internal/streams/streams.go:26 +0x134
main.main()
        /Users/svk/Documents/GitHub/go2rtc/main.go:46 +0x28
exit status 2

Now the configuration like this causing a crash at startup. this PR fix it

streams:
  - exec: "ffmpeg -i 'https://camera.lipetsk.ru/ms-25.camera.lipetsk.ru/live/6a3efc18-3651-43a4-b164-d07543151c3b/playlist.m3u8' -c:v libx264 -pix_fmt yuv420p -b:v 2368512 -f rtsp {output}"

[AlexxIT]

Thanks!

[AlexxIT]

https://github.com/AlexxIT/go2rtc/releases/tag/v1.9.0