mxfenc: don't try to write footer without header

This fixes a crash, when trying to mux h264 into mxf_opatom.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Previous version reviewed-by: tomas.hardin@codemill.se
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
