gst/mpegstream/: Fix for integer overflows in SCR discont detection. Workaround for broken incoming streams that enco...

Original commit message from CVS:
* gst/mpegstream/gstmpegdemux.c: (gst_mpeg_demux_send_discont),
(gst_mpeg_demux_parse_packet), (gst_mpeg_demux_send_subbuffer):
* gst/mpegstream/gstmpegparse.c: (gst_mpeg_parse_new_pad),
(gst_mpeg_parse_parse_packhead):
Fix for integer overflows in SCR discont detection. Workaround
for broken incoming streams that encounter apparent integer
overflows in server-side header rewriting. Some more debug.