Add bulletproofing in case stdin is closed.

Add bulletproofing in case stdin is closed.

(have_read_stdin): Remove global variable.
(dc_parse_stream): Always use stdin (freopen, if needed) rather
than sometimes using fopen to get a new file descriptor.
Call fclose unconditionally.
(main): Don't close stdin here.  If needed, now it's already done
by dc_parse_stream.