Roll libxml to 2.9.3

third_party/libxml/src/valid.c

 /*
  * valid.c : part of the code use to do the DTD handling and the validity
  *           checking
  *
  * See Copyright for the status of this software.
  *
  * daniel@veillard.com
  */
 
 #define IN_LIBXML
@@ skipping 2616 matching lines @@
         ret->attr = attr;
         ret->name = NULL;
     }
     ret->lineno = xmlGetLineNo(attr->parent);
 
     if (xmlHashAddEntry(table, value, ret) < 0) {
 #ifdef LIBXML_VALID_ENABLED
         /*
          * The id is already defined in this DTD.
          */
-        xmlErrValidNode(ctxt, attr->parent, XML_DTD_ID_REDEFINED,
-                        "ID %s already defined\n", value, NULL, NULL);
+        if (ctxt != NULL) {
+            xmlErrValidNode(ctxt, attr->parent, XML_DTD_ID_REDEFINED,
+                            "ID %s already defined\n", value, NULL, NULL);
+        }
 #endif /* LIBXML_VALID_ENABLED */
         xmlFreeID(ret);
         return(NULL);
     }
     if (attr != NULL)
         attr->atype = XML_ATTRIBUTE_ID;
     return(ret);
 }
 
 /**
@@ skipping 4394 matching lines @@
      */
     test_node->name = name;
     xmlFreeNode(test_node);
 
     return(nb_valid_elements);
 }
 #endif /* LIBXML_VALID_ENABLED */
 
 #define bottom_valid
 #include "elfgcchack.h"