stream_filter/httplive.c: NexSegment() look for next segment in *any* of the hls_stream_t's.
stream_filter/httplive.c: NexSegment() look for next segment in *any* of the hls_stream_t's.

NextSegment() assumed that the download thread its hls_stream_t was leading.
However this was too simplistic.

NextSegment() seeks the segment in *any* of the hls_stream_t, assuming that
each segment will be downloaded only once.

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