mxfdec: pass correct context to av_timecode_init()

fixes null ptr dereference
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1354,7 +1354,7 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
         if ((component = mxf_resolve_strong_ref(mxf, &material_track->sequence_ref, TimecodeComponent))) {
             mxf_tc = (MXFTimecodeComponent*)component;
             flags = mxf_tc->drop_frame == 1 ? AV_TIMECODE_FLAG_DROPFRAME : 0;
-            if (av_timecode_init(&tc, mxf_tc->rate, flags, mxf_tc->start_frame, mxf) == 0) {
+            if (av_timecode_init(&tc, mxf_tc->rate, flags, mxf_tc->start_frame, mxf->fc) == 0) {
                 mxf_add_timecode_metadata(&mxf->fc->metadata, "timecode", &tc);
             }
         }
@@ -1371,7 +1371,7 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
 
             mxf_tc = (MXFTimecodeComponent*)component;
             flags = mxf_tc->drop_frame == 1 ? AV_TIMECODE_FLAG_DROPFRAME : 0;
-            if (av_timecode_init(&tc, mxf_tc->rate, flags, mxf_tc->start_frame, mxf) == 0) {
+            if (av_timecode_init(&tc, mxf_tc->rate, flags, mxf_tc->start_frame, mxf->fc) == 0) {
                 mxf_add_timecode_metadata(&mxf->fc->metadata, "timecode", &tc);
                 break;
             }