Assertion abort on the EXPLAIN for a query containing a simple IN subquery

[Igor Babaev]

The following test case (a modification of the test case for bug #12838171 from mysql-5.6 code line) causes an assertion abort in mariadb-5.2 on the assertion
`!table || (!table->write_set || bitmap_is_set(table->write_set, field_index) || bitmap_is_set(table->vcol_set, [field_index))'field.cc:7017]

CREATE TABLE ot (
  col_int_nokey int(11),
  col_varchar_nokey varchar(1)
) ;
INSERT INTO ot VALUES (1,'x');
CREATE TABLE it1 (
   col_int_key int(11),
   col_varchar_key varchar(1),
   col_varchar_key2 varchar(1),
   KEY idx_cvk_cvk2_cik (col_varchar_key, col_varchar_key2, col_int_key),
   KEY idx_cvk_cik (col_varchar_key, col_int_key)
);
INSERT INTO it1 VALUES (NULL,'x','x'), (NULL,'f','f');

EXPLAIN
SELECT col_int_nokey FROM ot
WHERE (col_varchar_nokey, 'x') IN
              (SELECT col_varchar_key, col_varchar_key2 FROM it1);

The following EXPLAIN

EXPLAIN
SELECT col_int_nokey FROM ot
WHERE col_varchar_nokey IN
   (SELECT col_varchar_key FROM it2 WHERE col_int_key IS NULL);

 that uses table it2

CREATE TABLE it2(
  col_int_key int(11),
  col_varchar_key varchar(1),
  KEY idx_cvk_cik (col_varchar_key,col_int_key)
) ;
INSERT INTO it2 VALUES (NULL,'x'), (NULL,'f');

erroneously returns
"Impossible WHERE noticed after reading const tables".