Don't try to install the temporary sudoers file if we didn't edit it.

By default, visudo does not edit files in a #includedir.
Fixes a NULL pointer defef on GNU hurd; Bug #647

plugins/sudoers/visudo.c

@@ -266,6 +266,7 @@ main(int argc, char *argv[])
      */
     if (reparse_sudoers(editor, args, strict, quiet)) {
 	TAILQ_FOREACH(sp, &sudoerslist, entries) {
+	    if (sp->doedit)
 		(void) install_sudoers(sp, oldperms);
 	}
     }