Fix use of strdup on a NULL pointer (#63)

The following set of commands can rarely cause a memory fault
when auditing[*] is enabled, although most of the time it will
simply cause ksh to write '(null)' to the auditing file in place
of a tty name:

$ [ -e /etc/ksh_audit ] || echo "/tmp/ksh_auditfile;$(id -u)" | sudo tee /etc/ksh_audit;
$ v=$(ksh  2> /dev/null +o rc -ic $'getopts a:bc: opt --man\nprint $?')
$ cat /tmp/ksh_auditfile
1000;1593599493;(null); getopts a:bc: opt --man

This happens because strdup is used unconditionally on the pointer
returned by 'ttyname', which can be NULL if stderr is closed. This
then causes 'hp->tty' to be set to null, as strdup returns NULL.
See https://github.com/att/ast/issues/1028

src/cmd/ksh93/edit/history.c:
- Make strdup duplicate 'notty' instead of NULL to prevent
  crashes.

[*] https://blog.fpmurphy.com/2008/12/ksh93-auditing-and-accounting.html

src/cmd/ksh93/edit/history.c

@@ -395,7 +395,7 @@ retry:
 			if(fd>=0)
 			{
 				fcntl(fd,F_SETFD,FD_CLOEXEC);
-				hp->tty = strdup(ttyname(2));
+				hp->tty = strdup(isatty(2)?ttyname(2):"notty");
 				hp->auditfp = sfnew((Sfio_t*)0,NULL,-1,fd,SF_WRITE);
 			}
 		}