stream.hls: fix byterange parser

[bastimeyer]

Fixes #4300

https://datatracker.ietf.org/doc/html/rfc8216#section-4.3.2.2

The EXT-X-BYTERANGE tag indicates that a Media Segment is a sub-range
of the resource identified by its URI. It applies only to the next
URI line that follows it in the Playlist. Its format is:

#EXT-X-BYTERANGE:<n>[@<o>]

where n is a decimal-integer indicating the length of the sub-range
in bytes. If present, o is a decimal-integer indicating the start of
the sub-range, as a byte offset from the beginning of the resource.
If o is not present, the sub-range begins at the next byte following
the sub-range of the previous Media Segment.

If o is not present, a previous Media Segment MUST appear in the
Playlist file and MUST be a sub-range of the same media resource, or
the Media Segment is undefined and the client MUST fail to parse the
Playlist.

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I'll take a look at writing some tests for this once I get the time. I'm currently busy with other stuff. There's also the other issue which I've talked about in #4300 which needs to be fixed too.