Audio: Irregular timing shown at the last few seconds of the local playback.

Trying to get the "mPositionTimeMediaUs" when buffer size is zero resulted in unexpected time value.
Fix: Check to get the mediatime in fillbuffer only when there is non zero buffer, (When the input buffer is available).

Change-Id: Ifa12e761e3585a7d377d4aba2a2eb5674bcacf64
Author: Gurudatta Bhakte <gurudattax.bhakte@intel.com>
Signed-off-by: Gurudatta Bhakte <gurudattax.bhakte@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 24538