We have an assert that ensures that the end time is greater than
the start time for any cue. However we never checked that cues
had a non-zero duration when parsing them.
We will throw away cues with a duration of zero (and print a
warning message) as they are not spec compliant.
Closes: #335
Change-Id: I404e8f3a5a8d43eff75a2554db3e38e8d340f421