fix for a nasty bag, that causes a memory corruption+crash on HUGE select...

fix for a nasty bag, that causes a memory corruption+crash on HUGE select distincts, as myisam temp table treats nulls as equal, it constantly returned 141 (duplicate key) error in create_myisam_from_heap

sql/sql_select.cc

@@ -3885,7 +3885,7 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
       goto err;
     table->key_info=keyinfo;
     keyinfo->key_part=key_part_info;
-    keyinfo->flags=HA_NOSAME;
+    keyinfo->flags=HA_NOSAME|HA_NULL_ARE_EQUAL;
     keyinfo->key_length=(uint16) reclength;
     keyinfo->name=(char*) "tmp";
     if (null_pack_length)