clang warning fix (Richy).

git-svn-id: https://www.libavg.de/svn/trunk/libavg@8779 44470bb9-56e9-0310-a0f8-c586564d3dc6
Fixed build with newer ffmpeg (#391).

git-svn-id: https://www.libavg.de/svn/trunk/libavg@8780 44470bb9-56e9-0310-a0f8-c586564d3dc6