svg-sanitizer: Mixed-case xlink:HrEf skips the `<use>` nesting-DoS check in Resolver::processReferences

Summary Resolver::processReferences collects elements with the XPath predicate use@href or @xlink:href, which is case sensitive. A element written as xlink:HrEf is therefore never added to the reference graph, so the nesting-DoS nullification never marks it for removal...