Fix locations of NODE_UNDEF in undef with multiple args

* parse.y: Fix the beginning position of trailing NODE_UNDEF.

  e.g. The location of the NODE_UNDEF for `b` is fixed:

  ```
  undef a, b
  ```

  * Before

  ```
  NODE_UNDEF (line: 1, location: (1,6)-(1,10))
  ```

  * After

  ```
  NODE_UNDEF (line: 1, location: (1,9)-(1,10))
  ```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65973 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

parse.y

@@ -1784,7 +1784,7 @@ undef_list	: fitem
 		| undef_list ',' {SET_LEX_STATE(EXPR_FNAME|EXPR_FITEM);} fitem
 		    {
 		    /*%%%*/
-			NODE *undef = NEW_UNDEF($4, &@$);
+			NODE *undef = NEW_UNDEF($4, &@4);
 			$$ = block_append(p, $1, undef);
 		    /*% %*/
 		    /*% ripper: rb_ary_push($1, get_value($4)) %*/