Review fixes of new pushed code

- Fixed tests
- Optimized new code
- Fixed some unlikely core dumps
- Better bug fixes for:
  - #14397 - OPTIMIZE TABLE with an open HANDLER causes a crash
  - #14850 (ERROR 1062 when a quering a view using a Group By on a column that can be null

mysql-test/r/mysqlshow.result

@@ -1,4 +1,4 @@
-DROP TABLE IF EXISTS t1,t2;
+DROP TABLE IF EXISTS t1,t2,test1,test2;
 CREATE TABLE t1 (a int);
 INSERT INTO t1 VALUES (1),(2),(3);
 CREATE TABLE t2 (a int, b int);