Fixed segfault on listening socket close.

Now keep-alive connection sends 408 response if listening
socket was closed while reconfiguration.

src/nxt_conn_read.c

@@ -60,6 +60,8 @@ nxt_conn_io_read(nxt_task_t *task, void *obj, void *data)
 
         } else {
             n = state->io_read_handler(c);
+            /* The state can be changed by io_read_handler. */
+            state = c->read_state;
         }
 
         if (n > 0) {