script: avoid trying fclose(NULL)

script: avoid trying fclose(NULL)

Here is a one-liner to reproduce the issue.

$ mkdir example && cd example && chmod 0500 . && script
Script started, file is typescript
script: cannot open typescript: Permission denied
                                                 Script done, file is typescript
Segmentation fault (core dumped)

Addresses: https://bugs.launchpad.net/bugs/1537518
Signed-off-by: Sami Kerola <kerolasa@iki.fi>