Avoid a NULL dereference when handling a malformed fastcgi request.

Avoid a NULL dereference when handling a malformed fastcgi request.

Rework the hack to avoid a use-after-free in the fastcgi code.
Since server_fcgi() can be called by server_read_httpcontent() we
can't set clt_fcgi_error to NULL.  Instead, we implement a simple
reference count to track when a fastcgi session is in progress to
avoid closing the http session prematurely on fastcgi error.
Based on a diff from and OK by tb@.  Reported by Ben Kallus.