rtpdec_vp8: Make sure the previous packet is returned

This is a bug from c7d4de3d - 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>