gst/matroska/ebml-read.c: Use GINT64 formatting constants from GLIB.

gst/matroska/ebml-read.c: Use GINT64 formatting constants from GLIB.

Original commit message from CVS:
* gst/matroska/ebml-read.c:
Use GINT64 formatting constants from GLIB.
* gst/matroska/matroska-demux.c:
Add some guards to avoid a possible division by 0 and crashing
with NULL events on some systems.
Use gst_gdouble_to_guint64 somewhere instead of an implicit
conversion.
* gst/matroska/matroska-mux.c:
Check for invalid timestamps in a bunch of places to avoid
writing bogus durations into the output file.
Fix some double<->gint64 conversions that weren't using
gst_guint64_to_gdouble