Fixed AIO on Linux, broken in r4306.

Events from eventfd do not have c->write set, and the stale event
check added in r4306 causes null pointer dereference.

src/event/modules/ngx_epoll_module.c

diff --git a/src/event/modules/ngx_epoll_module.c b/src/event/modules/ngx_epoll_module.c
index 4f9c511..f4003af 100644
--- a/src/event/modules/ngx_epoll_module.c
+++ b/src/event/modules/ngx_epoll_module.c
@@ -681,20 +681,20 @@
 
         wev = c->write;
 
-        if (c->fd == -1 || wev->instance != instance) {
-
-            /*
-             * the stale event from a file descriptor
-             * that was just closed in this iteration
-             */
-
-            ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
-                           "epoll: stale event %p", c);
-            continue;
-        }
-
         if ((revents & EPOLLOUT) && wev->active) {
 
+            if (c->fd == -1 || wev->instance != instance) {
+
+                /*
+                 * the stale event from a file descriptor
+                 * that was just closed in this iteration
+                 */
+
+                ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
+                               "epoll: stale event %p", c);
+                continue;
+            }
+
             if (flags & NGX_POST_THREAD_EVENTS) {
                 wev->posted_ready = 1;