asvenc: free avctx->coded_frame on codec close
mxfdec: free descriptor extradata in .read_close()

Fixes memleak in fate-mxf-demux.
vp8: fix PPC assembly to work if src_stride != dst_stride

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Janne Grunau <janne-libav@jannau.net>