Fix handling of XML-1.0 XML namespace declaration

Usually 'xml' namespace for XML-1.0 declaration does not need
to be carried but Mike Hommey raised the problem that the SVG
XSD file fails to parse due to a mishandling.
- SAX2.c: failure to create a namespace should not be interpreted
  as a memory allocation error
- tree.c: document better xmlNewNs behaviour, and fix it in the
  case the 'xml' prefix is being used.

SAX2.c

@@ -2242,8 +2242,12 @@ xmlSAX2StartElementNs(void *ctx,
 	    if ((URI != NULL) && (prefix == pref))
 		ret->ns = ns;
 	} else {
-	    xmlSAX2ErrMemory(ctxt, "xmlSAX2StartElementNs");
-	    return;
+            /*
+             * any out of memory error would already have been raised
+             * but we can't be garanteed it's the actual error due to the
+             * API, best is to skip in this case
+             */
+	    continue;
 	}
 #ifdef LIBXML_VALID_ENABLED
 	if ((!ctxt->html) && ctxt->validate && ctxt->wellFormed &&

tree.c

@@ -721,8 +721,19 @@ xmlNewNs(xmlNodePtr node, const xmlChar *href, const xmlChar *prefix) {
     if ((node != NULL) && (node->type != XML_ELEMENT_NODE))
 	return(NULL);
 
-    if ((prefix != NULL) && (xmlStrEqual(prefix, BAD_CAST "xml")))
-	return(NULL);
+    if ((prefix != NULL) && (xmlStrEqual(prefix, BAD_CAST "xml"))) {
+        /* xml namespace is predefined, no need to add it */
+        if (xmlStrEqual(href, XML_XML_NAMESPACE))
+            return(NULL);
+
+        /*
+         * Problem, this is an attempt to bind xml prefix to a wrong
+         * namespace, which breaks
+         * Namespace constraint: Reserved Prefixes and Namespace Names
+         * from XML namespace. But documents authors may not care in
+         * their context so let's proceed.
+         */
+    }
 
     /*
      * Allocate a new Namespace and fill the fields.