Fix bug 1642: FTP assertion/blank page on authentication error

Fix bug 1642: FTP assertion/blank page on authentication error

http://www.squid-cache.org/bugs/show_bug.cgi?id=1642

* break out the connection completion code away from the FwdState destructor
* place into ::completed
* Add a flag which goes true when completed has been called; add some
  debugging to indicate (loudly!) if this condition is kicked off
* call ::completed via ::complete if the connection isn't being forwarded;
  mirroring the fwdStateFree() semantics in fwdComplete() from squid-2