hal: fix noise heard in single song repeat mode

Check state of the offload stream before calling flush on
the compress driver. It is possible that AF may
call flush from it's threadLoop without any active tracks.
It does so to clear state maintained within AudioStreamOut,
an intermediate object between AF and HAL.

Change-Id: Icf6b7484f163e46a8ddf3efbca1f780affd76ada
CRs-Fixed: 1055025