Decoders implementing the receive_frame API currently mostly use
stack packets to temporarily hold the packet they receive from
ff_decode_get_packet(). This role directly parallels the role of
in_pkt, the spare packet used in decode_simple_internal for the
decoders implementing the traditional decoding API. Said packet
is unused by the generic code for the decoders implementing the
receive_frame API, so allow them to use it to fulfill the function
it already fulfills for the traditional API for both APIs.

There is only one caveat in this: The packet is automatically
unreferenced in avcodec_flush_buffers(). But this is actually
positive as it means the decoders don't have to do this themselves
(in case the packet is preserved between receive_frame calls).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>