Prevent race in when stopping a "read thread"

Both StopReadThread and the thread being stopped set the thread id to
0 after m_read_thread_enabled was set to false. If the thread being
stopped got there first then StopReadThread called pthread_join on an
invalid thread number. This is not a Good Thing,

Should fix a fairly regular segfault when quitting on Linux.

llvm-svn: 185107