audioflinger: Fix monkey crash in audioflinger set parameter.

-Track ref ptr is initialized to direct track pointer only in create
 Direct Track although the descriptor is created in open Output
-If there is a set parameter coming in during this time it could
 result in a crash as track ref ptr is not initialized.
-Initialize track ref ptr to NULL when descriptor is created.

Change-Id: Iee887baee45a9e7b381f6e2ea2ab523a23689f6b