tree: Handle predefined entities in xmlBufGetEntityRefContent

It's possible to create references to predefined entities using the tree
API. This edge case was exposed by making predefined entities const in
commit 63ce5f9a.

testparser.c

@@ -59,6 +59,25 @@ testUnsupportedEncoding(void) {
     return err;
 }
 
+static int
+testNodeGetContent(void) {
+    xmlDocPtr doc;
+    xmlChar *content;
+    int err = 0;
+
+    doc = xmlReadDoc(BAD_CAST "<doc/>", NULL, NULL, 0);
+    xmlAddChild(doc->children, xmlNewReference(doc, BAD_CAST "lt"));
+    content = xmlNodeGetContent((xmlNodePtr) doc);
+    if (strcmp((char *) content, "<") != 0) {
+        fprintf(stderr, "xmlNodeGetContent failed\n");
+        err = 1;
+    }
+    xmlFree(content);
+    xmlFreeDoc(doc);
+
+    return err;
+}
+
 #ifdef LIBXML_SAX1_ENABLED
 static int
 testBalancedChunk(void) {
@@ -373,6 +392,7 @@ main(void) {
 
     err |= testStandaloneWithEncoding();
     err |= testUnsupportedEncoding();
+    err |= testNodeGetContent();
 #ifdef LIBXML_SAX1_ENABLED
     err |= testBalancedChunk();
 #endif