tree: Fix xmlTextMerge with NULL args

Restore pre-2.13 behavior. Fixes #875.

Author: nwellnhof

fuzz/api.c

@@ -2461,11 +2461,14 @@ LLVMFuzzerTestOneInput(const char *data, size_t size) {
                 first = getNode(0);
                 second = getNode(1);
                 argsOk =
-                    (first != NULL && first->type == XML_TEXT_NODE &&
-                     second != NULL && second->type == XML_TEXT_NODE &&
-                     first != second &&
-                     first->name == second->name);
-                if (argsOk) {
+                    first == NULL ?
+                        second != NULL :
+                        second == NULL ||
+                        (first->type == XML_TEXT_NODE &&
+                         second->type == XML_TEXT_NODE &&
+                         first != second &&
+                         first->name == second->name);
+                if (argsOk && second != NULL) {
                     if (second->parent != NULL)
                         parent = second->parent;
                     else
@@ -2474,7 +2477,7 @@ LLVMFuzzerTestOneInput(const char *data, size_t size) {
                 }
                 res = xmlTextMerge(first, second);
                 oomReport = (argsOk && res == NULL);
-                if (res != NULL) {
+                if (res != NULL && first != NULL) {
                     removeNode(second);
                     dropNode(parent);
                     checkContent(first);

tree.c

@@ -5790,15 +5790,21 @@ xmlNodeAddContent(xmlNodePtr cur, const xmlChar *content) {
  * @first:  the first text node
  * @second:  the second text node being merged
  *
- * Merge the second text node into the first. The second node is
- * unlinked and freed.
+ * Merge the second text node into the first. If @first is NULL,
+ * @second is returned. Otherwise, the second node is unlinked and
+ * freed.
  *
  * Returns the first text node augmented or NULL in case of error.
  */
 xmlNodePtr
 xmlTextMerge(xmlNodePtr first, xmlNodePtr second) {
-    if ((first == NULL) || (first->type != XML_TEXT_NODE) ||
-        (second == NULL) || (second->type != XML_TEXT_NODE) ||
+    if (first == NULL)
+        return(second);
+    if (second == NULL)
+        return(first);
+
+    if ((first->type != XML_TEXT_NODE) ||
+        (second->type != XML_TEXT_NODE) ||
         (first == second) ||
         (first->name != second->name))
 	return(NULL);