gst/mxf/mxfdemux.c: Properly combine flow returns of the source pads.

gst/mxf/mxfdemux.c: Properly combine flow returns of the source pads.

Original commit message from CVS:
* gst/mxf/mxfdemux.c: (gst_mxf_pad_init),
(gst_mxf_demux_reset_metadata), (gst_mxf_demux_combine_flows),
(gst_mxf_demux_handle_generic_container_essence_element):
Properly combine flow returns of the source pads.
Improve debugging and reset pad fields together with the
metadata to prevent invalid memory references.
* gst/mxf/mxfparse.c: (mxf_metadata_preface_parse):
Fix typo.