MDEV-20632: Fix -Wmaybe-uninitialized

Create_tmp_table::add_fields(): Initialize uneven_delta= 0
to suppress the warning from GCC 9.2.1 and 10.0.1,
and consistently indent the code with spaces.

sql/sql_select.cc

@@ -18345,7 +18345,6 @@ bool Create_tmp_table::add_fields(THD *thd,
   List_iterator_fast<Item> li(fields);
   Item *item;
   Field **tmp_from_field= m_from_field;
-  uint uneven_delta;
   while (!m_with_cycle && (item= li++))
     if (item->common_flags & IS_IN_WITH_CYCLE)
     {
@@ -18360,6 +18359,7 @@ bool Create_tmp_table::add_fields(THD *thd,
       distinct_record_structure= true;
     }
   li.rewind();
+  uint uneven_delta= 0;
   while ((item=li++))
   {
     current_counter= (((param->hidden_field_count < (fieldnr + 1)) &&