mroonga/storage.* tests fail with GCC 8

It seems that GCC8 causes some mroonga/storage.* tests to fail.

Failing tests:

• mroonga/storage.index_multiple_column_range_all_used_less_than
• mroonga/storage.index_multiple_column_range_all_used_less_than_or_equal
• mroonga/storage.index_multiple_column_range_partially_used_have_prefix_less_than
• mroonga/storage.index_multiple_column_range_partially_used_have_prefix_less_than_or_equal
• mroonga/storage.index_multiple_column_range_partially_used_no_prefix_less_than
• mroonga/storage.index_multiple_column_range_partially_used_no_prefix_less_than_or_equal
• mroonga/storage.optimization_order_limit_optimized_datetime_less_than
• mroonga/storage.optimization_order_limit_optimized_datetime_less_than_or_equal

I'm attaching 2 build logs: The first is for MariaDB built with GCC 7 (no mroonga/storage.* tests are failing), the second is the same MariaDB version built with GCC 8 (8 mroonga/storage.* tests are failing).

Btw. According to [1] (the list of tests that should be skipped) it seems that Fedora is facing the same problem.

[1] https://src.fedoraproject.org/rpms/mariadb/blob/master/f/rh-skipped-tests-base.list