`to_html_with_options("[][a \n]\n\n[a ]:\0", &Options::default())`
panics in 1.0.0 release at `src/subtokenize.rs:149` with `expected
link`. The 15-byte input is a collapsed-reference link `[][a \n]`
whose label contains a space-newline pair, paired with a definition
`[a ]:` whose url is a single NUL byte. The link-chain consumer
reaches an event whose `link` field is `None` and unconditionally
`expect`s it.

Same panic site as wooormGH-79 (`[](a (a \n))`) but distinct input shape:
this is a collapsed-reference link with NUL-byte url, not an inline
link with a quoted title. The empty `[]` text and the space-newline
in the label are both load-bearing. A single chain-builder integrity
fix in `src/construct/label_end.rs` likely closes both shapes
together.

Verified to fail against `markdown = "=1.0.0"` in release. Found via
in-tree fuzzing campaign.

Related-to: wooormGH-79.