gst/matroska/matroska-demux.c: Fix a bad conversion using gst_guint64_to_gdouble. fabs ((gdouble) demux->index[entry]...

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroskademux_do_index_seek):
Fix a bad conversion using gst_guint64_to_gdouble.
fabs ((gdouble) demux->index[entry].time - (gdouble) seek_pos) can not be
replaced by fabs (gst_guint64_to_gdouble (demux->index[entry].time - seek_pos)) as the
difference could be negative. fabs (gst_guint64_to_gdouble (demux->index[entry].time) -
gst_guint64_to_gdouble (seek_pos)) is the good solution. Thanks to Tim who has seen my
mistake.