Fix memory leak in syntax error in prism

If there is a syntax error, there could be an ast_node in the result.
This could get leaked if there is a syntax error so parsing could not
complete (parsed is not set to true).

For example, the following script leaks memory:

    10.times do
      10_000.times do
        eval("def foo(...) super(...) {}; end")
      rescue SyntaxError
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    31328
    42768
    53856
    65120
    76208
    86768
    97856
    109120
    120208
    131296

After:

    20944
    20944
    20944
    20944
    20944
    20944
    20944
    20944
    20944
    20944

prism_compile.c

@@ -10118,9 +10118,11 @@ pm_iseq_compile_node(rb_iseq_t *iseq, pm_scope_node_t *node)
 void
 pm_parse_result_free(pm_parse_result_t *result)
 {
-    if (result->parsed) {
+    if (result->node.ast_node != NULL) {
         pm_node_destroy(&result->parser, result->node.ast_node);
+    }
 
+    if (result->parsed) {
         xfree(result->node.constants);
         pm_scope_node_destroy(&result->node);
     }

test/ruby/test_syntax.rb

@@ -2204,6 +2204,20 @@ eom
     assert_syntax_error('def foo(...) super(1, ...) {}; end', /both block arg and actual block/)
   end
 
+  def test_argument_forwarding_with_super_memory_leak
+    assert_no_memory_leak([], "#{<<-'begin;'}", "#{<<-'end;'}", rss: true)
+      code = proc do
+        eval("def foo(...) super(...) {}; end")
+        raise "unreachable"
+      rescue SyntaxError
+      end
+
+      1_000.times(&code)
+    begin;
+      100_000.times(&code)
+    end;
+  end
+
   def test_class_module_Object_ancestors
     assert_separately([], <<-RUBY)
       m = Module.new