Description:
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Trying to unregister already unregistered nodeclass throws E_FATAL..

Unregistering nodeclass before any nodeclass is registered works fine however.

Reproduce code:
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<?php
class elm extends DOMNode {}

$dom = new DOMDocument;
// Unregistering nothing, works fine
$dom->registerNodeClass("DOMNode", null);

$dom->registerNodeClass("DOMNode", "elm");
// Unregister "elm", works fine
$dom->registerNodeClass("DOMNode", null);

// Unregistering nothing, E_FATAL
$dom->registerNodeClass("DOMNode", null);


Expected result:
----------------
Nothing to happen.

Actual result:
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Fatal error: DOMDocument::registerNodeClass(): Class (null) could not be registered. in /crypto/bjori/- on line 13