dashdemux: post-review fixup of UTCTiming element

dashdemux: post-review fixup of UTCTiming element

This commit addresses the following items from the code review:
    use a portable way to define NTP_TO_UNIX_EPOCH,
    fix memory leak on error, and
    add documentation to UTCTiming parse functions

Using LL is not portable, so the G_GUINT64_CONSTANT needs to be instead.

If an error occurs during DNS resolution, the GError was not being
released, causing a memory leak.

https://bugzilla.gnome.org/show_bug.cgi?id=752413