Explicit failure in VM_CHECK_MODE when failing to create timer thread

"warning: pthread_create failed for timer: Resource temporarily
unavailable, scheduling broken" still occurs randomly.  This change will
allow us to debug the issue.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61706 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_pthread.c

@@ -1603,6 +1603,7 @@ rb_thread_create_timer_thread(void)
 	if (err != 0) {
 	    rb_warn("pthread_attr_init failed for timer: %s, scheduling broken",
 		    strerror(err));
+	    VM_ASSERT(err == 0);
 	    return;
         }
 # ifdef PTHREAD_STACK_MIN