recv_input(input_fd, &inbuf) returns zero when @inbuf is full or
@input_fd is at EOF. We avoid the former by putting @input_fd in
@rdfd only when @inbuf has space, so we can detect EOF easily. But we
missed the case where adding a cookie fills up @inbuf. We
misinterpret "can't read into full buffer" as "EOF on input" then.
Fix by checking for space again.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>