mxf: Override faulty PreviousPartition entries

Some files set the PreviousPartition field to point to its own offset.

If we are parsing forward the Previous partition is immediately known
and its value could be used, otherwise we can safely point to the
header.
Reported-By: Jean Baptiste Kempf <jb@videolan.org>

libavformat/mxfdec.c

@@ -478,6 +478,26 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
     avio_read(pb, op, sizeof(UID));
     nb_essence_containers = avio_rb32(pb);
 
+    if (partition->this_partition &&
+        partition->previous_partition == partition->this_partition) {
+        av_log(mxf->fc, AV_LOG_ERROR,
+               "PreviousPartition equal to ThisPartition %"PRIx64"\n",
+               partition->previous_partition);
+        /* override with the actual previous partition offset */
+        if (!mxf->parsing_backward && mxf->last_forward_partition > 1) {
+            MXFPartition *prev =
+                mxf->partitions + mxf->last_forward_partition - 2;
+            partition->previous_partition = prev->this_partition;
+        }
+        /* if no previous body partition are found point to the header
+         * partition */
+        if (partition->previous_partition == partition->this_partition);
+            partition->previous_partition = 0;
+        av_log(mxf->fc, AV_LOG_ERROR,
+               "Overriding PreviousPartition with %"PRIx64"\n",
+               partition->previous_partition);
+    }
+
     /* some files don'thave FooterPartition set in every partition */
     if (footer_partition) {
         if (mxf->footer_partition && mxf->footer_partition != footer_partition) {