Revert "FFMpeg: Fix uninitialized AVFrame, which led to crashes with recent version of libav and ffmpeg"

This reverts commit 84903fce2548688ac47cb8b2d98e5876e3e55142.
Merge pull request #162 from teleshoes/fix_ffmpeg

Revert 84903fc: "FFMpeg: Fix uninitialized AVFrame"