Check for null EX(func) in write_property

This can happen if zend_call_function inserted a dummy frame,
and we already switched to the dummy frame in leave_helper,
and an exception i

Check for null EX(func) in write_property

This can happen if zend_call_function inserted a dummy frame,
and we already switched to the dummy frame in leave_helper,
and an exception is thrown during CV destruction.

Fixes oss-fuzz #25343.

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