Fix a segfault when replacing an unparented node.

The libxml2 function xmlReplaceNode will return NULL when the node to be
replaced has no parent.

Nokogiri now anticipates this and returns an exception instead of segfaulting.

This fixes issue #759, which includes a good script to reproduce and which
was reported by etm.

ext/nokogiri/xml_node.c

@@ -84,7 +84,7 @@ static xmlNodePtr xmlReplaceNodeWrapper(xmlNodePtr pivot, xmlNodePtr new_node)
   }
 
   /* work around libxml2 issue: https://bugzilla.gnome.org/show_bug.cgi?id=615612 */
-  if (retval->type == XML_TEXT_NODE) {
+  if (retval && retval->type == XML_TEXT_NODE) {
     if (retval->prev && retval->prev->type == XML_TEXT_NODE) {
       retval = xmlTextMerge(retval->prev, retval);
     }

test/xml/test_unparented_node.rb

@@ -403,6 +403,15 @@ def test_replace
         assert_equal set[0].to_xml, second.to_xml
       end
 
+      def test_replace_on_unparented_node
+        foo = Node.new('foo', @node.document)
+        if Nokogiri.jruby? # JRuby Nokogiri doesn't raise an exception
+          @node.replace(foo)
+        else
+          assert_raises(RuntimeError){ @node.replace(foo) }
+        end
+      end
+
       def test_illegal_replace_of_node_with_doc
         new_node = Nokogiri::XML.parse('<foo>bar</foo>')
         old_node = @node.at('.//employee')