avformat/utils: Avoid duplicating extradata from extract_extradata BSF

avformat/utils: Avoid duplicating extradata from extract_extradata BSF

Instead move the extradata contained in packet side-data to its
destination. This is possible because the side data already has zeroed
padding.

Notice that the check for FF_MAX_EXTRADATA_SIZE has been dropped,
because said constant is from libavcodec/internal.h. If libavcodec
wanted to enforce this, it should do so in the extract_extradata BSF
instead.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>