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, 4 deletions

diff --git a/sql/set_var.cc b/sql/set_var.cc
index 7bf6b9f928d..c9dfeb3e211 100644
--- a/sql/set_var.cc
+++ b/sql/set_var.cc
@@ -768,8 +768,7 @@ int set_var::check(THD *thd)
   if (!value)
     return 0;
 
-  if ((!value->fixed &&
-       value->fix_fields(thd, &value)) || value->check_cols(1))
+  if (value->fix_fields_if_needed_for_scalar(thd, &value))
     return -1;
   if (var->check_update_type(value))
   {
@@ -803,8 +802,7 @@ int set_var::light_check(THD *thd)
   if (type == OPT_GLOBAL && check_global_access(thd, SUPER_ACL))
     return 1;
 
-  if (value && ((!value->fixed && value->fix_fields(thd, &value)) ||
-                value->check_cols(1)))
+  if (value && value->fix_fields_if_needed_for_scalar(thd, &value))
     return -1;
   return 0;
 }