avformat/mxfenc: Discard audio until valid video has been received

avformat/mxfenc: Discard audio until valid video has been received

Normally, video packets are muxed before audio packets for mxf (there is
a dedicated interleave function for this); furthermore the first (video)
packet triggers writing the actual header. Yet when the first video packet
fails the checks performed on it, it will be an audio packet that leads
to writing the header and codec_ul (a value set based upon
properties of the bitstream which necessitates actually inspecting
packets) may be wrong. Therefore this commit discards audio packets until
a valid video packet has been received.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>