avfilter/vf_uspp: Fix leak of packet side data

avfilter/vf_uspp: Fix leak of packet side data

The uspp filter uses a special option ("no_bitstream") of
the Snow encoder to suppress it from generating output.
The filter therefore did not unref the packet after usage,
believing it to be blank. But this is wrong, as the Snow encoder
attaches quality stats side data to the packet.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>