Anchor link check breaks when the heading is wrapped as a link

[matthandus]

We found that the anchor link check reports dead links when the targeted heading is wrapped as its own link.

Working sample

An [anchor link](#heading-without-a-link) to a heading that passes, as expected.

### Heading without a link

Broken sample

An [anchor link](#heading-with-a-link) to a heading that reports as dead because the heading is wrapped in a link.

### [Heading with a link](https://github.com/tcort/markdown-link-check)