tree: Restore return value of xmlNodeListGetString with NULL list

When passing a NULL list to xmlNodeListGetString or
xmlNodeListGetRawString, return NULL instead of "" to match the old
behavior.

Fixes #783.

fuzz/api.c

@@ -2521,30 +2521,38 @@ LLVMFuzzerTestOneInput(const char *data, size_t size) {
             }
 
             case OP_XML_NODE_LIST_GET_STRING: {
+                xmlDocPtr doc;
+                xmlNodePtr list;
                 xmlChar *string;
 
                 startOp("xmlNodeListGetString");
                 incStrIdx();
+                doc = getDoc(0);
+                list = getNode(1);
                 string = xmlNodeListGetString(
-                    getDoc(0),
+                    doc,
-                    getNode(1),
+                    list,
                     getInt(0));
-                oomReport = (string == NULL);
+                oomReport = (list != NULL && string == NULL);
                 moveStr(0, string);
                 endOp();
                 break;
             }
 
             case OP_XML_NODE_LIST_GET_RAW_STRING: {
+                xmlDocPtr doc;
+                xmlNodePtr list;
                 xmlChar *string;
 
                 startOp("xmlNodeListGetRawString");
                 incStrIdx();
+                doc = getDoc(0);
+                list = getNode(1);
                 string = xmlNodeListGetRawString(
-                    getDoc(0),
+                    doc,
-                    getNode(1),
+                    list,
                     getInt(0));
-                oomReport = (string == NULL);
+                oomReport = (list != NULL && string == NULL);
                 moveStr(0, string);
                 endOp();
                 break;

tree.c

@@ -1559,11 +1559,11 @@ error:
 /**
  * xmlNodeListGetString:
  * @doc:  a document (optional)
- * @list:  a node list of attribute children (optional)
+ * @list:  a node list of attribute children
  * @inLine:  whether entity references are substituted
  *
  * Serializes attribute children (text and entity reference nodes)
- * into a string. An empty list produces an empty string.
+ * into a string.
  *
  * If @inLine is true, entity references will be substituted.
  * Otherwise, entity references will be kept and special characters
@@ -1577,11 +1577,14 @@ xmlNodeListGetString(xmlDocPtr doc, const xmlNode *list, int inLine)
 {
     int escMode;
 
+    /* backward compatibility */
+    if (list == NULL)
+        return(NULL);
+
     if (inLine) {
         escMode = 0;
     } else {
-        if ((list != NULL) &&
+        if ((list->parent != NULL) &&
-            (list->parent != NULL) &&
             (list->parent->type == XML_ATTRIBUTE_NODE))
             escMode = 2;
         else
@@ -1594,11 +1597,11 @@ xmlNodeListGetString(xmlDocPtr doc, const xmlNode *list, int inLine)
 /**
  * xmlNodeListGetRawString:
  * @doc:  a document (optional)
- * @list:  a node list of attribute children (optional)
+ * @list:  a node list of attribute children
  * @inLine:  whether entity references are substituted
  *
  * Serializes attribute children (text and entity reference nodes)
- * into a string. An empty list produces an empty string.
+ * into a string.
  *
  * If @inLine is true, entity references will be substituted.
  * Otherwise, entity references will be kept and special characters
@@ -1610,6 +1613,11 @@ xmlChar *
 xmlNodeListGetRawString(const xmlDoc *doc, const xmlNode *list, int inLine)
 {
     int escMode = inLine ? 0 : 3;
+
+    /* backward compatibility */
+    if (list == NULL)
+        return(NULL);
+
     return(xmlNodeListGetStringInternal((xmlDocPtr) doc, list, escMode));
 }
 

valid.c

@@ -6190,7 +6190,10 @@ xmlValidateElement(xmlValidCtxtPtr ctxt, xmlDocPtr doc, xmlNodePtr root) {
         if (elem->type == XML_ELEMENT_NODE) {
             attr = elem->properties;
             while (attr != NULL) {
-                value = xmlNodeListGetString(doc, attr->children, 0);
+                if (attr->children == NULL)
+                    value = xmlStrdup(BAD_CAST "");
+                else
+                    value = xmlNodeListGetString(doc, attr->children, 0);
                 if (value == NULL) {
                     xmlVErrMemory(ctxt);
                     ret = 0;

xmlreader.c

@@ -2279,6 +2279,8 @@ xmlTextReaderGetAttributeNo(xmlTextReaderPtr reader, int no) {
     }
     /* TODO walk the DTD if present */
 
+    if (cur->children == NULL)
+        return(NULL);
     ret = xmlNodeListGetString(reader->node->doc, cur->children, 1);
     if (ret == NULL)
         xmlTextReaderErrMemory(reader);
@@ -3524,6 +3526,8 @@ xmlTextReaderValue(xmlTextReaderPtr reader) {
             xmlDocPtr doc = NULL;
             xmlChar *ret;
 
+            if (attr->children == NULL)
+                return(NULL);
 	    if (attr->parent != NULL)
                 doc = attr->parent->doc;
 	    ret = xmlNodeListGetString(doc, attr->children, 1);