MDEV-16385 ROW SP variable is allowed in unexpected context

The problem described in the bug report happened because the code
did not test check_cols(1) after fix_fields() in a few places.

Additionally, fix_fields() could be called multiple times for SP variables,
because they are all fixed at a early stage in append_for_log().

Solution:
1. Adding a few helper methods
   - fix_fields_if_needed()
   - fix_fields_if_needed_for_scalar()
   - fix_fields_if_needed_for_bool()
   - fix_fields_if_needed_for_order_by()
  and using it in many cases instead of fix_fields() where
  the "fixed" status is not definitely known to be "false".

2. Adding DBUG_ASSERT(!fixed) into Item_splocal*::fix_fields()
   to catch double execution.

3. Adding tests.

As a good side effect, the patch removes a lot of duplicate code (~60 lines):

   if (!item->fixed &&
       item->fix_fields(..) &&
       item->check_cols(1))
     return true;

1 files changed, 35 insertions, 0 deletions

diff --git a/mysql-test/main/sp-row.test b/mysql-test/main/sp-row.test
index 837e24c89c0..3352c940cb6 100644
--- a/mysql-test/main/sp-row.test
+++ b/mysql-test/main/sp-row.test
@@ -1504,3 +1504,38 @@ BEGIN NOT ATOMIC
 END;
 $$
 DELIMITER ;$$
+
+--echo #
+--echo # MDEV-16385 ROW SP variable is allowed in unexpected context
+--echo #
+
+CREATE TABLE t1 (a INT);
+DELIMITER $$;
+--error ER_OPERAND_COLUMNS
+BEGIN NOT ATOMIC
+  DECLARE row ROW(a INT);
+  SELECT * FROM t1 ORDER BY row;
+END;
+$$
+DELIMITER ;$$
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT);
+DELIMITER $$;
+--error ER_OPERAND_COLUMNS
+BEGIN NOT ATOMIC
+  DECLARE row ROW(a INT);
+  SELECT * FROM t1 HAVING row;
+END;
+$$
+DELIMITER ;$$
+DROP TABLE t1;
+
+DELIMITER $$;
+--error ER_OPERAND_COLUMNS
+BEGIN NOT ATOMIC
+  DECLARE a ROW(a INT);
+  SELECT 1 LIKE 2 ESCAPE a;
+END;
+$$
+DELIMITER ;$$