The disconnect function should not complain when the server is already
closed by an explicit close.

Saw this failure in vtest:

*    top   TEST ../../../../bin/varnishtest/tests/r01918.vtc starting
...
**   s1    === txresp -nolen -bodylen 10
**** s1    txresp|HTTP/1.1 200 OK\r
**** s1    txresp|\r
**** s1    txresp|!"#$%&'()
**   s1    === close
...
**   s1    Waiting for server (4/-1)
**** s1    Closed
***  s1    shutting fd -1
---- s1    Shutdown failed: Bad file descriptor