Fix: keep ref to Node in XML::XPathContext

We need the document Node, but we also connect the Node to the xpath
context, so we should retain the ref to Node that will indirectly ref
the document Node.

Author: ysbaddaden

src/xml/xpath_context.cr

@@ -1,10 +1,9 @@
 class XML::XPathContext
   getter errors = [] of XML::Error
 
-  def initialize(node : Node)
-    @ctx = LibXML.xmlXPathNewContext(node.to_unsafe.value.doc)
-    @ctx.value.node = node.to_unsafe
-    @document = node.document
+  def initialize(@node : Node)
+    @ctx = LibXML.xmlXPathNewContext(@node.to_unsafe.value.doc)
+    @ctx.value.node = @node.to_unsafe
 
     {% if LibXML.has_method?(:xmlXPathSetErrorHandler) %}
       LibXML.xmlXPathSetErrorHandler(@ctx, ->Error.structured_callback, Box.box(@errors))
@@ -45,12 +44,12 @@ class XML::XPathContext
       when LibXML::XPathObjectType::NODESET
         if xpath_object.value.nodesetval
           # don't free xpath object: that would free the nodeset
-          return NodeSet.new(@document, xpath_object)
+          return NodeSet.new(@node.document, xpath_object)
         end
       end
 
     LibXML.xmlXPathFreeObject(xpath_object)
-    retval || NodeSet.new(@document)
+    retval || NodeSet.new(@node.document)
   end
 
   def register_namespaces(namespaces) : Nil