MDEV-14695: Assertion `n < m_size' failed in Bounds_checked_array<Element_type>::operator

In this issue we hit the assert because we are adding addition fields to the field JOIN::all_fields list. This
is done because  HEAP tables can't index BIT fields so  we need to use an additional hidden field for grouping because later it will be
converted to a LONG field. Original field will remain of the BIT type and will be returned. This happens when we convert DISTINCT to
GROUP BY.

The solution is to take into account the number of such hidden fields that would be added to the field
JOIN::all_fields list while calculating the size of the ref_pointer_array.

sql/sql_union.cc

@@ -1474,6 +1474,7 @@ bool st_select_lex::cleanup()
   }
   inner_refs_list.empty();
   exclude_from_table_unique_test= FALSE;
+  hidden_bit_fields= 0;
   DBUG_RETURN(error);
 }