Fixes #654.

Note references can be parsed during the reference-gathering pass,
before footnote ordering is initialized. Previously this could crash
with a NoMethodError for malformed reference labels.

`RDoc::Markdown.parse("[[^0]\n")`

Before the fix, it raised:
```ruby
NoMethodError: undefined method `<<' for nil
.../lib/rdoc/markdown.rb:...:in `note_for'
```
Or:
`RDoc::Markdown.parse("[foo[^1]bar]\n\n[^1]: footnote\n")`