[TD-6167]<fix> process stmt is null in stmt_close

src/client/src/tscPrepare.c

@@ -1781,8 +1781,8 @@ int taos_stmt_set_tbname(TAOS_STMT* stmt, const char* name) {
 
 int taos_stmt_close(TAOS_STMT* stmt) {
   STscStmt* pStmt = (STscStmt*)stmt;
-  if (pStmt == NULL){
-    STMT_RET(TSDB_CODE_SUCCESS);
+  if (stmt == NULL || pStmt->pSql == NULL || pStmt->taos == NULL) {
+    STMT_RET(TSDB_CODE_TSC_DISCONNECTED);
   }
   if (!pStmt->isInsert) {
     SNormalStmt* normal = &pStmt->normal;