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, 1 deletions

diff --git a/sql/sql_view.cc b/sql/sql_view.cc
index 6679334552b..783c655d007 100644
--- a/sql/sql_view.cc
+++ b/sql/sql_view.cc
@@ -1938,7 +1938,7 @@ bool check_key_in_view(THD *thd, TABLE_LIST *view)
     DBUG_PRINT("info", ("thd->column_usage: %d", thd->column_usage));
     for (Field_translator *fld= trans; fld < end_of_trans; fld++)
     {
-      if (!fld->item->fixed && fld->item->fix_fields(thd, &fld->item))
+      if (fld->item->fix_fields_if_needed(thd, &fld->item))
       {
         thd->column_usage= saved_column_usage;
         DBUG_RETURN(TRUE);