media: fix memory leak in audio decoder

AVPacket are not freed after use. The solution deals with
allocating AVPacket on the stack and ensuring to free it
calling av_packet_unref.
This fix has also widly deployed on Video for having a unique
method other the code.

Change-Id: I049b29effb4e6748ee2dd12b4c52536c99fef498
Tuleap: #91