Let the ubf_async_safe member of struct rb_vm_struct be a bit flag as…

… well

vm_core.h

@@ -702,7 +702,7 @@ typedef struct rb_vm_struct {
     struct ccan_list_head waiting_fds; /* <=> struct waiting_fd */
 
     /* set in single-threaded processes only: */
-    volatile int ubf_async_safe;
+    volatile unsigned int ubf_async_safe: 1;
 
     unsigned int running: 1;
     unsigned int thread_abort_on_exception: 1;