WIP: ensure we trap XML errors while applying XSLT stylesheet

otherwise trivial XML errors will silently result in NULL being
returned by xsltApplyStylesheet() and a subsequent segfault

WIP NOTE that we need a test here to prevent regressions. The test I have
available is too large to commit to the repo.

Fixes #1802

ext/nokogiri/xslt_stylesheet.c

@@ -165,7 +165,7 @@ static VALUE transform(int argc, VALUE* argv, VALUE self)
 
     errstr = rb_str_new(0, 0);
     xsltSetGenericErrorFunc((void *)errstr, xslt_generic_error_handler);
-    xmlSetGenericErrorFunc(NULL, (xmlGenericErrorFunc)&swallow_superfluous_xml_errors);
+    xmlSetGenericErrorFunc((void *)errstr, xslt_generic_error_handler);
 
     result = xsltApplyStylesheet(wrapper->ss, xml, params);
     free(params);
@@ -180,7 +180,11 @@ static VALUE transform(int argc, VALUE* argv, VALUE self)
       rb_exc_raise(exception);
     }
 
-    return Nokogiri_wrap_xml_document((VALUE)0, result) ;
+    if (result) {
+      return Nokogiri_wrap_xml_document((VALUE)0, result) ;
+    }
+
+    rb_raise(rb_eRuntimeError, "unknown error transforming document, please report as a bug");
 }
 
 static void method_caller(xmlXPathParserContextPtr ctxt, int nargs)