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>

term-utils/script.c

@@ -205,7 +205,8 @@ static void __attribute__((__noreturn__)) done(struct script_control *ctl)
 
 	if (ctl->timingfp)
 		fclose(ctl->timingfp);
-	fclose(ctl->typescriptfp);
+	if (ctl->typescriptfp)
+		fclose(ctl->typescriptfp);
 
 	if (ctl->rc_wanted) {
 		if (WIFSIGNALED(ctl->childstatus))