BUG/MAJOR: http: don't manipulate the server connection if it's killed

BUG/MAJOR: http: don't manipulate the server connection if it's killed

Jesse Hathaway reported a crash that Cyril Bonté diagnosed as being
caused by the manipulation of srv_conn after setting it to NULL. This
happens in http-server-close mode when the server returns either a 401
or a 407, because the connection was previously closed then it's being
assigned the CO_FL_PRIVATE flag.

This bug only affects 1.6-dev as it was introduced by connection reuse code
with commit 387ebf8 ("MINOR: connection: add a new flag CO_FL_PRIVATE").