lavf/mux: do not pass a copy of the packet to write_packet().

Sometimes the muxer modifies the packet, like for instance lavf/mp3enc
changing pkt->destruct in order to keep a copy. These changes must be
kept, even though the muxer behaviour is questionable. Regression since
0072116.

Fixes #2124.
dirac: fix inverted check

Regression since: ea6da80
Fixes Ticket2123

I cannot reproduce any regressions by flipping the wrong condition
to how it should have been.

Thanks-to: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>