Add couple of missing Null checks

For https://bugzilla.gnome.org/show_bug.cgi?id=733710
Reported by Gaurav but with slightly different fixes
diff --git a/tree.c b/tree.c
index 7853e35..7d41481 100644
--- a/tree.c
+++ b/tree.c
@@ -4514,6 +4514,10 @@
 #ifdef LIBXML_TREE_ENABLED
     if (doc->intSubset != NULL) {
         ret->intSubset = xmlCopyDtd(doc->intSubset);
+	if (ret->intSubset == NULL) {
+	    xmlFreeDoc(ret);
+	    return(NULL);
+	}
 	xmlSetTreeDoc((xmlNodePtr)ret->intSubset, ret);
 	ret->intSubset->parent = ret;
     }