[#3443] Fix IllegalStateException which could be triggered when the channel goes inactive during the eventloop processing

Motivation:

This is a regression that was introduced as part of 6b941e9bdbc1b1a9090c280bc6c44903ff7c7b67. The regression could produce an "infinity" triggering of IllegalStateException if a channel goes inactive while process the events for it.

Modifications:

Correctly check if the channel is still active before trigger the callbacks.

Result:

No more IllegalStateException

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollEventLoop.java

@@ -310,17 +310,24 @@ final class EpollEventLoop extends SingleThreadEventLoop {
                     boolean write = (ev & Native.EPOLLOUT) != 0;
 
                     AbstractEpollUnsafe unsafe = (AbstractEpollUnsafe) ch.unsafe();
-                    if (write) {
+
+                    if (close) {
+                        unsafe.epollRdHupReady();
+                    }
+
+                    // We need to check if the channel is still open before try to trigger the
+                    // callbacks as otherwise we may trigger an IllegalStateException when try
+                    // to access the file descriptor.
+                    //
+                    // See https://github.com/netty/netty/issues/3443
+                    if (write && ch.isOpen()) {
                         // force flush of data as the epoll is writable again
                         unsafe.epollOutReady();
                     }
-                    if (read) {
+                    if (read && ch.isOpen()) {
                         // Something is ready to read, so consume it now
                         unsafe.epollInReady();
                     }
-                    if (close) {
-                        unsafe.epollRdHupReady();
-                    }
                 } else {
                     // We received an event for an fd which we not use anymore. Remove it from the epoll_event set.
                     Native.epollCtlDel(epollFd, fd);