DIOS-4906 Add correct AVFrame release to fix memory leak

[akola-dolby]

• Removed AV_BUFFERSRC_FLAG_KEEP_REF to avoid frame copy inside (buffer was not copied) (reference: https://ffmpeg.org/doxygen/trunk/group__lavfi__buffersrc.html#ga8fc71cb48c1ad1aa78b48f87daa4cf19).
• Replaced av_frame_unref to av_frame_free which removes a frame and a buffer, not only a buffer.

[CLAassistant]

All committers have signed the CLA.