Resolve markdown crash for invalid note reference

Issue originally reported here:
#654

Author: skatkov

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
+    raise ParseError, "invalid note reference: #{ref}" 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
+    raise ParseError, "invalid note reference: #{ref}" unless @note_order
+
     @note_order << ref
 
     label = @note_order.length

test/rdoc/rdoc_markdown_test.rb

@@ -1002,6 +1002,16 @@ def test_parse_note_inline
     assert_equal expected, doc
   end
 
+  def test_parse_note_invalid_reference
+    @parser.notes = true
+
+    error = assert_raise RDoc::Markdown::ParseError do
+      parse "[[^0]\n"
+    end
+
+    assert_equal "invalid note reference: 0", error.message
+  end
+
   def test_parse_note_no_notes
     @parser.notes = false