Fix Mutex#unlock with a scheduler and thread contention

* It would hit "[BUG] unexpected THREAD_STOPPED" before.

test/fiber/test_mutex.rb

@@ -47,6 +47,29 @@ class TestFiberMutex < Test::Unit::TestCase
     thread.join
   end
 
+  def test_mutex_thread
+    mutex = Mutex.new
+    mutex.lock
+
+    thread = Thread.new do
+      scheduler = Scheduler.new
+      Thread.current.scheduler = scheduler
+
+      Fiber.schedule do
+        mutex.lock
+        sleep 0.1
+        mutex.unlock
+      end
+
+      scheduler.run
+    end
+
+    sleep 0.1
+    mutex.unlock
+
+    thread.join
+  end
+
   def test_condition_variable
     mutex = Mutex.new
     condition = ConditionVariable.new

thread_sync.c

@@ -402,8 +402,7 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_fiber_t *fiber)
 
             if (cur->th->scheduler != Qnil) {
                 rb_scheduler_unblock(cur->th->scheduler, cur->self, rb_fiberptr_self(cur->fiber));
-            }
-
+            } else {
                 switch (cur->th->status) {
                   case THREAD_RUNNABLE: /* from someone else calling Thread#run */
                   case THREAD_STOPPED_FOREVER: /* likely (rb_mutex_lock) */
@@ -417,6 +416,7 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_fiber_t *fiber)
                     continue;
                 }
             }
+	}
       found:
 	while (*th_mutex != mutex) {
 	    th_mutex = &(*th_mutex)->next_mutex;