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/sql_prepare.cc b/sql/sql_prepare.cc
index d4c96ebc535..fd526f9a660 100644
--- a/sql/sql_prepare.cc
+++ b/sql/sql_prepare.cc
@@ -1656,8 +1656,7 @@ static bool mysql_test_call_fields(Prepared_statement *stmt,
 
   while ((item= it++))
   {
-    if ((!item->fixed && item->fix_fields(thd, it.ref())) ||
-        item->check_cols(1))
+    if (item->fix_fields_if_needed_for_scalar(thd, it.ref()))
       goto err;
   }
   DBUG_RETURN(FALSE);
@@ -2665,8 +2664,7 @@ end:
 
 bool LEX::get_dynamic_sql_string(LEX_CSTRING *dst, String *buffer)
 {
-  if (prepared_stmt_code->fix_fields(thd, NULL) ||
-      prepared_stmt_code->check_cols(1))
+  if (prepared_stmt_code->fix_fields_if_needed_for_scalar(thd, NULL))
     return true;
 
   const String *str= prepared_stmt_code->val_str(buffer);