The following sequence was problematic:
1. See start of a cluster having not output a seek map. Decide
to seek for the cues. Enter CUES_STATE_BUILDING state.
2. Error occurs before seek map is output.
3. ExtractorSampleSource isn't prepared yet, so restarts from the
start of the stream.
4. See start of the same cluster having not output a seek map.
This time cuesState is CUES_STATE_BUILDING, so we just carry
on. We then fill the buffer with sample data, despite the
source not being prepared, at which point we get stuck.
It's unclear to me why cuesState needed three states, so I've rm'd
the BUILDING state. Step (4) above will now do the same thing as
in step (1). If the failure repeats, we'll eventually fail, which
is WAI.
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