rtpdec_vp8: Set the timestamp when returning a deferred packet

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpdec_vp8: Make sure the previous packet is returned

This is a bug from c7d4de3d73 - if the previous frame wasn't
returned yet (due to missing the final packets), but we have
enough data of it to return the first partition, we write that into
pkt and set returned_old_frame. That commit forgot returning 0 for
the case where this current packet didn't have the end_packet flag
set.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpdec_vp8: Avoid a warning about a possibly unused variable

The warning is a false positive, but I prefer actually initializing
it over masking it with av_uninit, since the code is not performance
critical.

Signed-off-by: Martin Storsjö <martin@martin.st>