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, 1 insertions, 2 deletions

diff --git a/sql/sql_trigger.cc b/sql/sql_trigger.cc
index 6961f821a0a..df3a2dc9c04 100644
--- a/sql/sql_trigger.cc
+++ b/sql/sql_trigger.cc
@@ -797,8 +797,7 @@ bool Table_triggers_list::create_trigger(THD *thd, TABLE_LIST *tables,
     */
     trg_field->setup_field(thd, table, NULL);
 
-    if (!trg_field->fixed &&
-        trg_field->fix_fields(thd, (Item **)0))
+    if (trg_field->fix_fields_if_needed(thd, (Item **)0))
       DBUG_RETURN(true);
   }