FFMPEG Media Plugin: Fixed some crashes due to missing initialization of fTempPacket fields and due to freeing memory we don't have ownership upon.

Signed-off-by: Colin Günther <coling@gmx.de>
Build configuration: Build posix part of libroot with debug symbols, to facilitate research of memory access exceptions in the FFMPEG Media Plugin.

Signed-off-by: Colin Günther <coling@gmx.de>