RTSPSource: check audio buffer count

For some special server, the audio AUs are too sparse, the duration of
each audio AU is too long. So there is often 1 or 0 audio AU in cache,
and causes cache duration check failure.
This change checks audio buffer count for sufficient data, so there will
always be enough buffers when buffering finishes.

CRs-Fixed: 818203
Change-Id: I8ce472a336d09a9d8fc3028ed1e9fab717e2e53c