Minor bug fixes to Balancer.

Minor bug fixes to Balancer.

This is a follow-up to the refactor done in
5726c9ceb0644af360d37b86351b97ddfcbee20c which prevents a crash when
the first entry in the stack happens to be a BalanceMarker (and thus
doesn't have a `$localName` property).  It also fixes an unrelated
issue where unpaired close-heading tags (like `</h3>`) get entity-escaped
instead of ignored.

Test cases exposing these bugs are added in
Ie854cf99f7e72bcca1bb8565ace558a43dcb6379.

Change-Id: Ia9a1d435be1be10512071f5ff626b68742863483