libavformat/mux, mxfenc: Don't initialize unnecessarily

libavformat/mux, mxfenc: Don't initialize unnecessarily

When no packet could be output, the interleavement functions
nevertheless initialized the packet destined for output (with the
exception of the data and size fields, making the initialization
pointless), although it will not be used at all. So remove the
initializations.

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>