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_help.cc b/sql/sql_help.cc
index 085e54dbc90..aa9f3fedd6d 100644
--- a/sql/sql_help.cc
+++ b/sql/sql_help.cc
@@ -611,8 +611,7 @@ int send_variant_2_list(MEM_ROOT *mem_root, Protocol *protocol,
 SQL_SELECT *prepare_simple_select(THD *thd, Item *cond,
 				  TABLE *table, int *error)
 {
-  if (!cond->fixed)
-    cond->fix_fields(thd, &cond);	// can never fail
+  cond->fix_fields_if_needed(thd, &cond);  // can never fail
 
   /* Assume that no indexes cover all required fields */
   table->covering_keys.clear_all();