adaptivdemux/hlsdemux2: Fix issues with failure updating playlists
This MR is to fix a range of issues that occured when dealing with (mostly HLS) streams on which some/all variants would fail.
- Don't reset the download error count when re-trying a failed playlist
- Handle loop/context issues resulting from stopping the demuxer while the manifest is being processed
- Don't use
g_str_equal
on potentially NULL strings