Fix pthread_detach for already-exited threads.

Change-Id: I2bf7f41234d93b226132a4c51705f4186f4961c3
Reported-by: Paresh Nakhe <pnakhe@codeaurora.org>

libc/bionic/pthread_detach.cpp

@@ -44,6 +44,12 @@ int pthread_detach(pthread_t t) {
     return 0; // Already being joined; silently do nothing, like glibc.
   }
 
+  if (thread->tid == 0) {
+    // Already exited; clean up.
+    _pthread_internal_remove_locked(thread.get());
+    return 0;
+  }
+
   thread->attr.flags |= PTHREAD_ATTR_FLAG_DETACHED;
   return 0;
 }