matroskademux: Fix cluster searching if we search multiple times in one chunk

After finding a cluster id in the byte reader, we skip ahead the reader
position by one further byte to be able to continue searching from there
inside the same chunk if the cluster candidate was a false positive.

We have to accomodate for that additional byte when resuming the search,
otherwise all following pulls are off-by-one for every resume and we run
into an assertion.

gst/matroska/matroska-demux.c

@@ -1766,13 +1766,22 @@ gst_matroska_demux_search_cluster (GstMatroskaDemux * demux, gint64 * pos)
       demux->common.offset += length + needed;
       ret = gst_matroska_read_common_peek_id_length_pull (&demux->common,
           GST_ELEMENT_CAST (demux), &id, &length, &needed);
-      if (ret != GST_FLOW_OK)
+      if (ret != GST_FLOW_OK) {
+        /* we skipped one byte in the reader above, need to accomodate for
+         * that when resuming skipping from the reader instead of reading a
+         * new chunk */
+        newpos += 1;
         goto resume;
+      }
       GST_DEBUG_OBJECT (demux, "next element is %scluster",
           id == GST_MATROSKA_ID_CLUSTER ? "" : "not ");
       if (id == GST_MATROSKA_ID_CLUSTER)
         break;
-      /* not ok, resume */
+      /* not ok, resume
+       * we skipped one byte in the reader above, need to accomodate for
+       * that when resuming skipping from the reader instead of reading a
+       * new chunk */
+      newpos += 1;
       goto resume;
     } else {
       /* partial cluster id may have been in tail of buffer */