prev_mn_schedulable might be clobbered by longjmp

thread.c

@@ -1811,7 +1811,7 @@ rb_thread_io_blocking_call(rb_blocking_function_t *func, void *data1, int fd, in
     volatile VALUE val = Qundef; /* shouldn't be used */
     volatile int saved_errno = 0;
     enum ruby_tag_type state;
-    bool prev_mn_schedulable = th->mn_schedulable;
+    volatile bool prev_mn_schedulable = th->mn_schedulable;
     th->mn_schedulable = thread_io_mn_schedulable(th, events, NULL);
 
     // `errno` is only valid when there is an actual error - but we can't