Fixed an issue caused by e4263d306e6988dd322c895242eb818d22b9e012.

This sets `stateInHtmlBlock` to `Just "div"` when we're parsing
an HTML div.

Without this fix, a closing `</div>` tag could be parsed as part
of a list item rather than after the list.
HTML writer:  Deactivate "incremental" inside slide speaker notes.

Closes #1394.