Fix SummaryParser::parse_title() consuming events (#1743)

[ilslv]

Resolves #1743

The issue can be found while looking and summary parsing output:

#
- [First](./first.md)
  - [Nested](./nested.md)
- [Second](./second.md)

RUST_LOG=mdbook::book::summary=TRACE cargo run -- build

2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Next event: Some(Start(Heading(H1, None, [])))
2022-02-11 11:52:10 [DEBUG] (mdbook::book::summary): Found a h1 in the SUMMARY
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Next event: Some(End(Heading(H1, None, [])))
2022-02-11 11:52:10 [DEBUG] (mdbook::book::summary): Parsing prefix items
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:10 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 11:52:10 [DEBUG] (mdbook::book::summary): Parsing numbered chapters at level 0
...

---

- [First](./first.md)
  - [Nested](./nested.md)
- [Second](./second.md)

RUST_LOG=mdbook::book::summary=TRACE cargo run -- build

2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:46 [DEBUG] (mdbook::book::summary): Parsing prefix items
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Start(Item))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Start(Link(Inline, Borrowed("./first.md"), Borrowed(""))))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Text(Borrowed("First")))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(End(Link(Inline, Borrowed("./first.md"), Borrowed(""))))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 11:52:46 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 11:52:46 [DEBUG] (mdbook::book::summary): Parsing numbered chapters at level 0
...

By looking at first 2 lines of the output, we can see that Some(Start(List(None))) event isn't pushed back and so is skipped. This causes parser to see - [First](./first.md) as [First](./first.md) and so interprets it as prefix chapter.

After applying a fix we can see following output:

2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 12:16:38 [DEBUG] (mdbook::book::summary): Parsing prefix items
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Next event: Some(Start(List(None)))
2022-02-11 12:16:38 [TRACE] (mdbook::book::summary): Back: Start(List(None))
2022-02-11 12:16:38 [DEBUG] (mdbook::book::summary): Parsing numbered chapters at level 0
...

[ehuss]

Thanks!