matroskademux: Unmap wavpack header buffer after creating it

Otherwise it will be mapped writable all the time and we can't read from it
anywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=762239

gst/matroska/matroska-demux.c

@@ -2872,6 +2872,7 @@ gst_matroska_demux_add_wvpk_header (GstElement * element,
     GST_WRITE_UINT8 (data + 11, wvh.index_no);
     GST_WRITE_UINT32_LE (data + 12, wvh.total_samples);
     GST_WRITE_UINT32_LE (data + 16, wvh.block_index);
+    gst_buffer_unmap (newbuf, &outmap);
 
     /* Append data from buf: */
     gst_buffer_copy_into (newbuf, *buf, GST_BUFFER_COPY_TIMESTAMPS |