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, 2 insertions, 3 deletions

diff --git a/sql/unireg.cc b/sql/unireg.cc
index a9b47c72a4f..6540e11578b 100644
--- a/sql/unireg.cc
+++ b/sql/unireg.cc
@@ -1082,9 +1082,8 @@ static bool make_empty_rec(THD *thd, uchar *buff, uint table_options,
          field->real_field_type() == MYSQL_TYPE_GEOMETRY))
     {
       Item *expr= field->default_value->expr;
-
-      int res= !expr->fixed && // may be already fixed if ALTER TABLE
-                expr->fix_fields(thd, &expr);
+      // may be already fixed if ALTER TABLE
+      int res= expr->fix_fields_if_needed(thd, &expr);
       if (!res)
         res= expr->save_in_field(regfield, 1);
       if (!res && (field->flags & BLOB_FLAG))