Fix segv when parsing `command` by ripper

89cfc15207 made this event dispatch to pass `Qundef`
to user defined callback method by mistake.
This commit fix it to be `nil`.

parse.y

@@ -3447,7 +3447,7 @@ command		: fcall command_args       %prec tLOWEST
                     {
                         set_embraced_location($5, &@4, &@6);
                         $$ = new_command_qcall(p, idCOLON2, $1, $3, Qnull, $5, &@3, &@$);
-                    /*% ripper: method_add_block!(command_call!($:1, $:2, $:3, Qundef), $:5) %*/
+                    /*% ripper: method_add_block!(command_call!($:1, $:2, $:3, Qnil), $:5) %*/
                    }
                 | keyword_super command_args
                     {

test/ripper/test_sexp.rb

@@ -123,6 +123,21 @@ eot
     assert_equal(exp, named)
   end
 
+  def test_command
+    sexp = Ripper.sexp("a::C {}")
+    assert_equal(
+      [:program,
+        [
+          [:method_add_block,
+            [:command_call,
+              [:vcall, [:@ident, "a", [1, 0]]],
+              [:@op, "::", [1, 1]],
+              [:@const, "C", [1, 3]],
+              nil],
+          [:brace_block, nil, [[:void_stmt]]]]]],
+      sexp)
+  end
+
   def search_sexp(sym, sexp)
     return sexp if !sexp or sexp[0] == sym
     sexp.find do |e|