If ff_interleave_add_packet failed, the content of the newly created
packet new_pkt would leak.

Also, it is unnecessary to zero-initialize a packet that will be put
into av_new_packet lateron as the latter already initializes the packet.
Therefore this has been removed, too.
Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>