Work around race condition between thread shutdown and vendor lib shutdown.

This race condition occurs because of a structural bug - the userial
thread depends on the HCI thread and the HCI thread depends on the
userial thread but both are independently owned and torn down. This
change fixes a crash bug due to the race condition (NULL pointer
access in userial thread).

http://b/16483216

Change-Id: I91ea274856ac72e9d72b92f0dc5c94e53aaf22f4