LP:640419 - Wrong result with sort_union/index_merge in maria-5.1 and a large table

The following query:

SELECT * FROM `table100000_innodb_int_autoinc` WHERE ( ( `col_bigint_key` >= 1 ) OR `pk` BETWEEN 8 AND 40 + 64 ) OR ( `pk` BETWEEN 1 AND 1 + 185 AND `col_bigint_key` IS NULL )

Returns a different number of rows when executed with an index_merge/sort_union strategy.