FFMPEG Plugin: Refactor out checking of invalid conditions in audio path.

- Main reason for this refactoring is to increase readability and thus make
  audio decode path more comprehensible.

- Added documentation for the new method accordingly.

- Small change in calculating the decoded data size to clear when error occurs
  during decoding. This way it is more readable and more consistent with
  calculations of decoded data size on other locations.

- No functional change intended.