[libxml2] Revert "Support xmlTextReaderNextSibling w/o preparsed doc"

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 6fc04d714a019cb3be351bc472f7a64a08f51008
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Fri Nov 23 22:56:03 2018 +0100

    Revert "Support xmlTextReaderNextSibling w/o preparsed doc"
    
    This reverts commit bfec41b3de1cbd35e547b57c80ae3a5101f8891c which
    caused problems with the XML::LibXML Perl bindings.
    
    https://mail.gnome.org/archives/xml/2018-November/msg00010.html

 xmlreader.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)
---
diff --git a/xmlreader.c b/xmlreader.c
index 47a8b813..db310c8f 100644
--- a/xmlreader.c
+++ b/xmlreader.c
@@ -2027,6 +2027,7 @@ found_node:
  *
  * Skip to the node following the current one in document order while
  * avoiding the subtree if any.
+ * Currently implemented only for Readers built on a document
  *
  * Returns 1 if the node was read successfully, 0 if there is no more
  *          nodes to read, or -1 in case of error
@@ -2035,16 +2036,16 @@ int
 xmlTextReaderNextSibling(xmlTextReaderPtr reader) {
     if (reader == NULL)
         return(-1);
+    if (reader->doc == NULL) {
+        /* TODO */
+       return(-1);
+    }
 
     if (reader->state == XML_TEXTREADER_END)
         return(0);
 
-    if (reader->node == NULL) {
-        if (reader->doc != NULL)
-            return(xmlTextReaderNextTree(reader));
-        else
-            return(xmlTextReaderRead(reader));
-    }
+    if (reader->node == NULL)
+        return(xmlTextReaderNextTree(reader));
 
     if (reader->node->next != NULL) {
         reader->node = reader->node->next;