sys/oss/gstosssink.c: get_delay() may return values lower than 0. In those cases, we should not actually cast to *uns...

Original commit message from CVS:
* sys/oss/gstosssink.c: (gst_osssink_get_delay),
(gst_osssink_get_time):
get_delay() may return values lower than 0. In those cases, we
should not actually cast to *unsigned* int64, that will break
stuff horribly. In my case, it screwed up A/V sync in movies
in totem rather badly.