audio: Fix crash in flinger during monkey runs

The trackrefptr in DirectTrack descriptor can be invalid and
this can result in a crash is stability runs. Add check for
validity of the trackrefptr before accessing it.

Change-Id: I4c7fbff022f381f9d2cdbf0fc04ade8defbb2195
(cherry picked from commit 66917496a28241bf38f117cb05b5810021a185a0)
(cherry picked from commit e1c916bb324f4f6f7923b8e4b2a9c436a65534ff)
(cherry picked from commit dc983d3efd2a5ec3435246d4d2a2e1b4265b5f22)