backported MERGE+MAX bugfix from 4.0

sql/opt_sum.cc

@@ -165,10 +165,9 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
 	    error=table->file->index_last(table->record[0]) !=0;
 	  else
 	  {
-	    (void) table->file->index_read(table->record[0], key_buff,
-					   ref.key_length,
-					   HA_READ_AFTER_KEY);
-	    error=table->file->index_prev(table->record[0]) ||
+           error = table->file->index_read(table->record[0], key_buff,
+                                          ref.key_length,
+                                         HA_READ_PREFIX_LAST) ||
 	      key_cmp(table,key_buff,ref.key,ref.key_length);
 	  }
 	  if (table->key_read)