[TD-6167]<fix> add null pointer protection

src/client/src/tscPrepare.c

@@ -1986,7 +1986,6 @@ TAOS_RES *taos_stmt_use_result(TAOS_STMT* stmt) {
     return NULL;
   }
   TAOS_RES* result = pStmt->pSql;
-  pStmt->pSql = NULL;
   return result;
 }
 

tests/script/api/stmt.c

@@ -468,7 +468,6 @@ void taos_stmt_use_result_query(void *taos, char *col, int type) {
   assert(taos_stmt_close(stmt) == 0);
   free(params);
   free(stmt_sql);
-  taos_free_result(result);
 }
 
 void taos_stmt_use_result_test() {