avformat/mxfdec: dont ask for samples with field dominance 0 anymore

Based on discussion and patch from
"[FFmpeg-devel] [PATCH]Do not ask for mxf samples with unknown field dominance"
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1771,6 +1771,7 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
                         avpriv_request_sample(mxf->fc,
                                               "Field dominance %d support",
                                               descriptor->field_dominance);
+                    case 0: // we already have many samples with field_dominance == unknown
                         break;
                     }
                     /* Turn field height into frame height. */