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, 3 insertions, 10 deletions

diff --git a/sql/sql_signal.cc b/sql/sql_signal.cc
index 7a28ba65ba2..a92d40f6bb3 100644
--- a/sql/sql_signal.cc
+++ b/sql/sql_signal.cc
@@ -218,16 +218,9 @@ int Sql_cmd_common_signal::eval_signal_informations(THD *thd, Sql_condition *con
        i <= LAST_DIAG_SET_PROPERTY;
        i++)
   {
-    set= m_set_signal_information.m_item[i];
-    if (set)
-    {
-      if (! set->fixed)
-      {
-        if (set->fix_fields(thd, & set))
-          goto end;
-        m_set_signal_information.m_item[i]= set;
-      }
-    }
+    if ((set= m_set_signal_information.m_item[i]) &&
+        set->fix_fields_if_needed(thd, &m_set_signal_information.m_item[i]))
+      goto end;
   }
 
   /*