parse.y: Change locations of NODE_DVAR in NODE_MASGN

* parse.y: Change the last location of NODE_DVAR to
  be equal to the first location of NODE_DVAR.
  NODE_DVAR of NODE_MASGN (nd_value) is an internal variable,
  so it has no length.

  e.g. The locations of the NODE_DVAR is changed:

  ```
  a.b {|(c,d)| e}
  ```

  * Before

  ```
  NODE_DVAR (line: 1, code_range: (1,7)-(1,10))
  ```

  * After

  ```
  NODE_DVAR (line: 1, code_range: (1,7)-(1,7))
  ```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61124 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

parse.y

@@ -4643,13 +4643,16 @@ f_arg_item	: f_arg_asgn
 		| tLPAREN f_margs rparen
 		    {
 			ID tid = internal_id();
+			YYLTYPE location;
+			location.first_loc = @2.first_loc;
+			location.last_loc = @2.first_loc;
 			arg_var(tid);
 		    /*%%%*/
 			if (dyna_in_block()) {
-			    $2->nd_value = new_dvar(tid, &@2);
+			    $2->nd_value = new_dvar(tid, &location);
 			}
 			else {
-			    $2->nd_value = new_lvar(tid, &@2);
+			    $2->nd_value = new_lvar(tid, &location);
 			}
 			$$ = NEW_ARGS_AUX(tid, 1);
 			$$->nd_next = $2;