framework/av: Check for NULL pointer of mCblk and zero division error

latency calculation happens in two places: AudioTrack::latency() and
AudioTrack::dump(). Need to add null pointer and zero division check on
both of these places.

Change-Id: I9cbe8c1c00a390cd626bdbf202124f27e60eb3e3
Fixed-CRs: 548514