Fix: missing mutex

src/crystal/system/unix/epoll/event_loop.cr

@@ -77,7 +77,7 @@ class Crystal::Epoll::EventLoop < Crystal::Evented::EventLoop
 
     Crystal.trace :evloop, "wait", blocking: blocking ? 1 : 0
 
-    if blocking && (time = @timers.next_ready?)
+    if blocking && (time = @mutex.synchronize { @timers.next_ready? })
       # epoll_wait only has milliseconds precision, so we use a timerfd for
       # timeout; arm it to the next ready time
       @timerfd.set(time)
@@ -192,7 +192,7 @@ class Crystal::Epoll::EventLoop < Crystal::Evented::EventLoop
   end
 
   # unsafe, yields when there are no more events for fd
-  private def system_sync(node : Evented::EventQueue::Node) : Nil
+  private def system_sync(node : Evented::EventQueue::Node, &) : Nil
     events = 0
     events |= LibC::EPOLLIN if node.readers?
     events |= LibC::EPOLLOUT if node.writers?