Commit 81306fd4bdf ("hls: eliminate ffurl_* usage", merged in d0fc5de3)
changed the hls demuxer to use AVIOContext instead of URLContext for its
HTTP requests.

HLS demuxer uses the "offset" option of the http demuxer, requesting
the initial file offset for the I/O (http URLProtocol uses the "Range:"
HTTP header to try to accommodate that).

However, the code in libavformat/aviobuf.c seems to be doing its own
accounting for the current file offset (AVIOContext.pos), with the
assumption that the initial offset is always zero.

HLS demuxer does an explicit seek after open_url to account for cases
where the "offset" was not effective (due to the URL being a local file
or the HTTP server not obeying it), which should be a no-op in case the
file offset is already at that position.

However, since aviobuf.c code thinks the starting offset is 0, this
doesn't work properly.

This breaks retrieval of ranged media segments.

To fix the regression, just drop the seek call from the HLS demuxer when
the HTTP(S) protocol is used.