Resolve markdown crash for note references in labels

lib/rdoc/markdown.kpeg

@@ -403,6 +403,8 @@
   # the note order list for proper display at the end of the document.
 
   def note_for ref
+    return unless @note_order
+
     @note_order << ref
 
     label = @note_order.length

lib/rdoc/markdown.rb

@@ -788,6 +788,8 @@ def note label
   # the note order list for proper display at the end of the document.
 
   def note_for ref
+    return unless @note_order
+
     @note_order << ref
 
     label = @note_order.length

test/rdoc/rdoc_markdown_test.rb

@@ -1002,6 +1002,29 @@ def test_parse_note_inline
     assert_equal expected, doc
   end
 
+  def test_parse_note_invalid_reference
+    @parser.notes = true
+
+    assert_kind_of RDoc::Markup::Document, parse("[[^0]\n")
+  end
+
+  def test_parse_note_reference_in_reference_label
+    @parser.notes = true
+
+    doc = parse <<~MD
+      [foo[^1]bar]
+
+      [^1]: footnote
+    MD
+
+    expected = doc(
+      para("[foo{*1}[rdoc-label:foottext-1:footmark-1]bar]"),
+      rule(1),
+      para("{^1}[rdoc-label:footmark-1:foottext-1] footnote"))
+
+    assert_equal expected, doc
+  end
+
   def test_parse_note_no_notes
     @parser.notes = false