blast: fix crash TransactionCompletedThread

When TransactionCompletedThread is destroyed, the std::thread
is signaled to exit and then joined. If the thread was never
started, the join call throws an exception. To prevent the
exception, check if the thread is joinable first.

Test: libsurfaceflinger_unittest
Change-Id: I6372bf4fff4bc04383a9018dac01b36d1c046001