istream/dechunk: set the "closed" flag only in _Close()
istream/dechunk: set the "closed" flag only in _Close()

This flag is checked only where Istream::Close() is the only way to
set it.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy