Crash in HttpTunnel::consumer_reenable

Crash happened on 9.0.1.
In `HttpTunnel::consumer_reenable`, `c->write_vio` was null on this line.
```
NetVConnection *netvc = dynamic_cast(c->write_vio->vc_server);
```

```
#0 0x0000556c505a7172 in HttpTunnel::consumer_reenable(HttpTunnelConsumer*) (this=0x7fa1fd715b00, c=0x7fa1fd715b48)
#1 0x0000556c505a6a32 in HttpTunnel::consumer_handler(int, HttpTunnelConsumer*) (this=0x7fa1fd715b00, event=3, c=0x7fa1fd715b48)
#2 0x0000556c505a3fa3 in HttpTunnel::main_handler(int, void*) (this=0x7fa1fd715b00, event=4194304, data=)
#3 0x0000556c5071bcee in handleEvent (this=, event=3, data=0x7fffffffffffffff)
#4 write_signal_and_update(int, UnixNetVConnection*) (event=3, vc=0x7fa1af626bc0)
#5 0x0000556c5071b297 in write_signal_done (event=3, nh=0x7fac1a211290, vc=0x7fa1af626bc0)
#6 write_signal_error (nh=0x7fac1a211290, vc=0x7fa1af626bc0, lerrno=0)
#7 write_to_net_io(NetHandler*, UnixNetVConnection*, EThread*) (nh=0x7fac1a211290, vc=0x7fa1af626bc0, thread=)
#8 0x0000556c5071430c in NetHandler::process_ready_list() (this=0x7fac1a211290)
#9 0x0000556c50714a12 in NetHandler::waitForActivity(long) (this=, timeout=)
#10 0x0000556c50714aed in non-virtual thunk to NetHandler::waitForActivity(long) ()
#11 0x0000556c5074b70f in EThread::execute_regular() (this=0x7fac1a20d1c0)
#12 0x0000556c5074b91a in EThread::execute() (this=0x7fac1a20d1c0)
```