Snippet generation breaks with fuzzy fields

[mrcnski]

Describe the bug

• I added a fuzzy field to the query parser.
• Snippet generation started returning "".
• I would expect it to return snippets around the matched fuzzy terms.

Which version of tantivy are you using?
4aa8cd247

To Reproduce

I wrote the following test case in snippet/mod.rs (based on test_snippet_generator):

#[test]
fn test_snippet_generator_with_fuzzy_field() -> crate::Result<()> {
    let mut schema_builder = Schema::builder();
    let text_options = TextOptions::default().set_indexing_options(
        TextFieldIndexing::default()
            .set_tokenizer("en_stem")
            .set_index_option(IndexRecordOption::Basic),
    );
    let text_field = schema_builder.add_text_field("text", text_options);
    let schema = schema_builder.build();
    let index = Index::create_in_ram(schema);
    {
        // writing the segment
        let mut index_writer = index.writer_for_tests()?;
        let doc = doc!(text_field => TEST_TEXT);
        index_writer.add_document(doc)?;
        index_writer.commit()?;
    }
    let searcher = index.reader().unwrap().searcher();
    let mut query_parser = QueryParser::for_index(&index, vec![text_field]);
    query_parser.set_field_fuzzy(text_field, false, 1, true); // <--------------- THIS LINE ADDED <----
    let query = query_parser.parse_query("rust design").unwrap();
    let mut snippet_generator =
        SnippetGenerator::create(&searcher, &*query, text_field).unwrap();
    {
        let snippet = snippet_generator.snippet(TEST_TEXT);
        assert_eq!(
            snippet.to_html(),
            "imperative-procedural paradigms. <b>Rust</b> is syntactically similar to \
             C++[according to whom?],\nbut its <b>designers</b> intend it to provide better \
             memory safety"
        );
    }
    {
        snippet_generator.set_max_num_chars(90);
        let snippet = snippet_generator.snippet(TEST_TEXT);
        assert_eq!(
            snippet.to_html(),
            "<b>Rust</b> is syntactically similar to C++[according to whom?],\nbut its \
             <b>designers</b> intend it to"
        );
    }
    Ok(())
}

Output is:

---- snippet::tests::test_snippet_generator_with_fuzzy_field stdout ----
thread 'snippet::tests::test_snippet_generator_with_fuzzy_field' panicked at src/snippet/mod.rs:800:13:
assertion `left == right` failed
  left: ""
 right: "imperative-procedural paradigms. <b>Rust</b> is syntactically similar to C++[according to whom?],\nbut its <b>designers</b> intend it to provide better memory safety"