thread_pthread (rb_timer_arm): ignore UBF_TIMER_POSIX state 2

It looks like I forgot to account for a situation involving 3
threads.

[ruby-core:88360] [Misc #14937]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64354 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_pthread.c

@@ -1386,7 +1386,13 @@ rb_timer_arm(rb_pid_t current) /* async signal safe */
 
           case 1: return; /* success */
           case 2:
-            rb_async_bug_errno("UBF_TIMER_POSIX state 2 unexpected", EINVAL);
+            /*
+             * it is possible to have another thread disarm, and
+             * a third thread arm finish re-arming before we get
+             * here, so we wasted a syscall with timer_settime but
+             * probably unavoidable in a signal handler.
+             */
+            return;
           default:
             rb_async_bug_errno("UBF_TIMER_POSIX unknown state", ERANGE);
         }