(#524): Don't panic upon an XML processing instruction with a disallowed URL
XmlState.error() takes a mutable borrow on the inner RefCell, and it was already mutably borrowed by the caller XmlState.processing_instruction().
Fixes #524 (closed)
XmlState.error() takes a mutable borrow on the inner RefCell, and it was already mutably borrowed by the caller XmlState.processing_instruction().
Fixes #524 (closed)