Fix GCC lint complaint of calling NULL->cloneTree. No functional change intended.

src/V3Ast.cpp

@@ -646,7 +646,7 @@ AstNode* AstNode::cloneTreeIterList() {
 }
 
 AstNode* AstNode::cloneTree(bool cloneNextLink) {
-    if (!this) return NULL;  // verilog.y relies on this
+    UDEBUGONLY(UASSERT(dynamic_cast<AstNode*>(this),"this should not be NULL"););
     this->debugTreeChange("-cloneThs: ", __LINE__, cloneNextLink);
     cloneClearTree();
     AstNode* newp;