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/item_row.cc b/sql/item_row.cc
index 74ea9ecc0c2..8233ba00f06 100644
--- a/sql/item_row.cc
+++ b/sql/item_row.cc
@@ -41,8 +41,7 @@ bool Item_row::fix_fields(THD *thd, Item **ref)
   Item **arg, **arg_end;
   for (arg= args, arg_end= args + arg_count; arg != arg_end ; arg++)
   {
-    if (!(*arg)->fixed &&
-        (*arg)->fix_fields(thd, arg))
+    if ((*arg)->fix_fields_if_needed(thd, arg))
       return TRUE;
     // we can't assign 'item' before, because fix_fields() can change arg
     Item *item= *arg;