set Segment key error

[Ruk1ng001]

I get an error when all segments is nil in file reader.go:

        if state.tagKey {
            p.Segments[p.last()].Key = &Key{state.xkey.Method, state.xkey.URI, state.xkey.IV, state.xkey.Keyformat, state.xkey.Keyformatversions}
            // First EXT-X-KEY may appeared in the header of the playlist and linked to first segment
            // but for convenient playlist generation it also linked as default playlist key
            if p.Key == nil {
                p.Key = state.xkey
            }
            state.tagKey = false
        }

All the segment in p.Segments is nil, so cann't find p.Segments[p.last()].Key.