avisynth: Use av_packet_from_data instead of av_new_packet

If the audio changes from 9eac7c4 were merged as they were, this
would cause scripts with both video+audio to fail with a lot of
audio decoding errors (the video would be fine). Scripts with
only one of either video or audio were unaffected. Additionally,
the av_packet changes in general caused seeking to break.

Using av_packet_from_data allows video+audio scripts to work as
expected, without audio decoding errors.  It also fixes seeking.