fix benign data race in pthread_attr_init

access to defaults should be protected against concurrent changes.

src/thread/pthread_attr_init.c

@@ -6,7 +6,9 @@ extern size_t __default_guardsize;
 int pthread_attr_init(pthread_attr_t *a)
 {
 	*a = (pthread_attr_t){0};
+	__acquire_ptc();
 	a->_a_stacksize = __default_stacksize;
 	a->_a_guardsize = __default_guardsize;
+	__release_ptc();
 	return 0;
 }