Fixing listen event connection leakage.

A connection object is allocated in advance for each listen event object to be
used for the established connection.  This connection needs to be freed when the
listen event is destroyed.

src/nxt_router.c

@@ -3178,6 +3178,10 @@ nxt_router_listen_event_release(nxt_task_t *task, nxt_listen_event_t *lev,
     nxt_debug(task, "listen event count: %D", lev->count);
 
     if (--lev->count == 0) {
+        if (lev->next != NULL) {
+            nxt_conn_free(task, lev->next);
+        }
+
         nxt_free(lev);
     }