gst/mxf/mxfdemux.c: Don't allow negative or zero edit rates to prevent division by zero and we support negative edit ...

Original commit message from CVS:
* gst/mxf/mxfdemux.c:
(gst_mxf_demux_handle_header_metadata_update_streams),
(gst_mxf_demux_pad_next_component):
Don't allow negative or zero edit rates to prevent division
by zero and we support negative edit rates anyway.