HTML API: Prevent fragment creation on close tag

[sirreal]

Prevent fragments from being created at tag closers. This was noticed by @ockham in #7348:

This got me thinking -- should we disallow create_fragment_at_current_node from being called when the processor is paused on a tag closer?

(If we do, then we should be able to remove this guard, as it would be guaranteed that we're at a tag opener.)

Follow-up [59444]
Trac ticket: https://core.trac.wordpress.org/ticket/62357

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[cbravobernal]

Committed in https://core.trac.wordpress.org/changeset/59450.