Fix for core dump in DISTINCT

Docs/manual.texi

@@ -39222,6 +39222,9 @@ though, so Version 3.23 is not released as a stable version yet.
 @appendixsubsec Changes in release 3.23.29
 @itemize @bullet
 @item
+Fixed bug when doing a @code{SELECT DISTINCT ... table1 LEFT JOIN
+table2..] when table2 was empty.
+@item
 Added @code{--abort-slave-event-count} and
 @code{--disconnect-slave-event-count} options to @code{mysqld} for
 debugging and testing of replication

sql/sql_select.cc

@@ -5218,8 +5218,10 @@ remove_duplicates(JOIN *join, TABLE *entry,List<Item> &fields)
   List_iterator<Item> it(fields);
   Item *item;
   while ((item=it++))
-    if (item->tmp_table_field())
+  {
+    if (item->tmp_table_field() && ! item->const_item())
       field_count++;
+  }
 
   if (!field_count)
   {						// only const items