istream-pipe: clear handler before closing the input
istream-pipe: clear handler before closing the input

Prevent calling pipe_close() twice - first by pipe_abort(), then by
pipe_input_abort().


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