mxfdec: Check for NULL component

This fixes SIGSEGV with zzuf1.mxf

libavformat/mxfdec.c

@@ -1276,7 +1276,7 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
                 break;
             }
         }
-        if (!source_track)
+        if (!source_track || !component)
             continue;
 
         if (!(source_track->sequence = mxf_resolve_strong_ref(mxf, &source_track->sequence_ref, Sequence))) {