MDEV-6830: Server crashes in best_access_path after a sequence of SELECTs ...

generate_derived_keys_for_table() did not work correctly in the case where
- it had a potential index on derived table
- however, TABLE::check_tmp_key() would disallow creation of this index 
  after looking at its future key parts (because of the key parts exceeding 
  max. index length)
- the code would leave a KEYUSE structure that refers to a non-existant index.
  Depending on further optimizer calculations, this could cause a crash. 

mysql-test/r/mdev6830.result

+drop table if exists t1,t2,t3;
+drop view if exists v2,v3;
+CREATE TABLE t1 (pk INT PRIMARY KEY) ENGINE=MyISAM;
+CREATE TABLE t2 (
+f1 DATE,
+f2 VARCHAR(1024),
+f3 VARCHAR(10),
+f4 DATE,
+f5 VARCHAR(10),
+f6 VARCHAR(10),
+f7 VARCHAR(10),
+f8 DATETIME,
+f9 INT,
+f10 VARCHAR(1024),
+f11 VARCHAR(1024),
+f12 INT,
+f13 VARCHAR(1024)
+) ENGINE=MyISAM;
+CREATE OR REPLACE VIEW v2 AS SELECT * FROM t2;
+CREATE TABLE t3 (
+f1 VARCHAR(1024),
+f2 VARCHAR(1024),
+f3 DATETIME,
+f4 VARCHAR(10),
+f5 INT,
+f6 VARCHAR(10),
+f7 VARCHAR(1024),
+f8 VARCHAR(10),
+f9 INT,
+f10 DATE,
+f11 INT,
+f12 VARCHAR(1024),
+f13 VARCHAR(10),
+f14 DATE,
+f15 DATETIME
+) ENGINE=MyISAM;
+CREATE OR REPLACE ALGORITHM=TEMPTABLE VIEW v3 AS SELECT * FROM t3;
+INSERT INTO t3 VALUES  
+('FOO','foo','2000-08-04 00:00:00','one',1,'1','FOO','foo',1,'2004-05-09',1,'one','one','2001-12-07','2001-10-17 08:25:04'),  
+('BAR','bar','2001-01-01 04:52:37','two',2,'2','BAR','bar',2,'2008-01-01',2,'two','two','2006-06-19','2002-01-01 08:22:49');
+CREATE TABLE t4 (f1 VARCHAR(10), f2 INT) ENGINE=MyISAM;
+SELECT * FROM t1;
+pk
+SELECT non_existing FROM v2;
+ERROR 42S22: Unknown column 'non_existing' in 'field list'
+SELECT * FROM t1, v3, t4 WHERE v3.f1 = t4.f1 AND t4.f2 = 6 AND t1.pk = v3.f5;
+pk	f1	f2	f3	f4	f5	f6	f7	f8	f9	f10	f11	f12	f13	f14	f15	f1	f2
+drop table t1,t2,t3,t4;
+drop view v2,v3;

mysql-test/t/mdev6830-master.opt

+--debug

mysql-test/t/mdev6830.test

+
+--source include/have_debug.inc
+
+--disable_warnings
+drop table if exists t1,t2,t3;
+drop view if exists v2,v3;
+--enable_warnings
+CREATE TABLE t1 (pk INT PRIMARY KEY) ENGINE=MyISAM;
+
+CREATE TABLE t2 (
+  f1 DATE,
+  f2 VARCHAR(1024),
+  f3 VARCHAR(10),
+  f4 DATE,
+  f5 VARCHAR(10),
+  f6 VARCHAR(10),
+  f7 VARCHAR(10),
+  f8 DATETIME,
+  f9 INT,
+  f10 VARCHAR(1024),
+  f11 VARCHAR(1024),
+  f12 INT,
+  f13 VARCHAR(1024)
+) ENGINE=MyISAM;
+
+CREATE OR REPLACE VIEW v2 AS SELECT * FROM t2;
+
+CREATE TABLE t3 (
+  f1 VARCHAR(1024),
+  f2 VARCHAR(1024),
+  f3 DATETIME,
+  f4 VARCHAR(10),
+  f5 INT,
+  f6 VARCHAR(10),
+  f7 VARCHAR(1024),
+  f8 VARCHAR(10),
+  f9 INT,
+  f10 DATE,
+  f11 INT,
+  f12 VARCHAR(1024),
+  f13 VARCHAR(10),
+  f14 DATE,
+  f15 DATETIME
+) ENGINE=MyISAM;
+
+CREATE OR REPLACE ALGORITHM=TEMPTABLE VIEW v3 AS SELECT * FROM t3;
+
+INSERT INTO t3 VALUES  
+  ('FOO','foo','2000-08-04 00:00:00','one',1,'1','FOO','foo',1,'2004-05-09',1,'one','one','2001-12-07','2001-10-17 08:25:04'),  
+  ('BAR','bar','2001-01-01 04:52:37','two',2,'2','BAR','bar',2,'2008-01-01',2,'two','two','2006-06-19','2002-01-01 08:22:49');
+
+CREATE TABLE t4 (f1 VARCHAR(10), f2 INT) ENGINE=MyISAM;
+
+SELECT * FROM t1;
+
+--error ER_BAD_FIELD_ERROR
+SELECT non_existing FROM v2;
+
+SELECT * FROM t1, v3, t4 WHERE v3.f1 = t4.f1 AND t4.f2 = 6 AND t1.pk = v3.f5;
+
+drop table t1,t2,t3,t4;
+drop view v2,v3;
+

sql/sql_select.cc

@@ -9060,7 +9060,7 @@ bool generate_derived_keys_for_table(KEYUSE *keyuse, uint count, uint keys)
       else
       {
         /* Mark keyuses for this key to be excluded */
-        for (KEYUSE *curr=save_first_keyuse; curr < first_keyuse; curr++)
+        for (KEYUSE *curr=save_first_keyuse; curr < keyuse; curr++)
 	{
           curr->key= MAX_KEY;
         }