avformat/hls: fix segment selection regression on track changes of live streams

Commit ad701326 ("avformat/hls: open playlists immediately when
AVDISCARD_ALL is dropped") inadvertently caused first_packet to never be
cleared, causing select_cur_seq_no() to not use the specific code for
live streams.

In practice this means that when the user selects a different audio
track during live stream (i.e. non-VOD) playback, there may be some
additional delay as the code might select an incorrect segment at first,
and we have to wait for video to catch audio (if too late segment was
selected) or to download more following audio segments (if too early
segment was selected).

Fix that by restoring the zeroing of first_packet.