Make a copy of the current sudoers path when assigning errorfile.

Fixes a potential use after free in visudo when there is an error
in one of the include files.

plugins/sudoers/sudoers.h

@@ -266,7 +266,7 @@ int pam_prep_user(struct passwd *);
 /* gram.y */
 int sudoersparse(void);
 extern char *login_style;
-extern const char *errorfile;
+extern char *errorfile;
 extern int errorlineno;
 extern bool parse_error;
 extern bool sudoers_warnings;