avcodec/decode: Update decode_simple_internal() to get the side data correctly.

Use avci->last_pkt_props to get the side data. Using |pkt| doesn't work
when FF_API_MERGE_SD is set, as the compressed side data is expanded into
|tmp|, leaving the original |pkt| unchanged.

Signed-off-by: James Almer <jamrial@gmail.com>