Bug #17896: MIN of CASE WHEN returns non-minimum value!

- after review fixes

sql/item_cmpfunc.cc

@@ -39,19 +39,23 @@ static void agg_result_type(Item_result *type, Item **items, uint nitems)
 {
   Item **item, **item_end;
 
-  /* Note: NULL items don't affect the result type */
   *type= STRING_RESULT;
   /* Skip beginning NULL items */
   for (item= items, item_end= item + nitems; item < item_end; item++)
+  {
     if ((*item)->type() != Item::NULL_ITEM)
     {
       *type= (*item)->result_type();
       item++;
       break;
     }
+  }
+  /* Combine result types. Note: NULL items don't affect the result */
   for (; item < item_end; item++)
+  {
     if ((*item)->type() != Item::NULL_ITEM)
       *type= item_store_type(type[0], (*item)->result_type());
+  }
 }
 
 static void agg_cmp_type(Item_result *type, Item **items, uint nitems)