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

lldb/source/Core/Communication.cpp

@@ -390,8 +390,6 @@ Communication::ReadThread (void *p)
 
     // Let clients know that this thread is exiting
     comm->BroadcastEvent (eBroadcastBitReadThreadDidExit);
-    comm->m_read_thread_enabled = false;
-    comm->m_read_thread = LLDB_INVALID_HOST_THREAD;
     return NULL;
 }