FFMPEG Plugin: Refactor scope of fTempPacket in audio path.

- There are two main reasons for this refactoring:
    1. Prepare using FFMPEGs functionality of audio frame start time assignment
       (instead of rolling it ourself) like already done for the video path
       (see _LoadNextVideoChunkIfNeededAndAssignStartTime() for reference).
    2. Get rid of fChunkBufferOffset (this is a minor reason though).

- Untangle some of the conditional checks to increase readability.

- No functional change intended.