[PATCH] bigint fail with gcc 5.0

Originally spotted with mysql-5.6.24 (http://bugs.mysql.com/bug.php?id=76654), but mariadb 10.0.17 seems to have the same issue:

Description:
When building mysql-5.6.24 with gcc 5.0 on Fedora 22 or later some tests fail on big integers operations, for example bigint test case.

Full build log is available here:
https://kojipkgs.fedoraproject.org//work/tasks/3420/9443420/build.log

How to repeat:
Run bigint test or a small reproducer:

  MySQL [(none)]> select -(-9223372036854775808);

  +-------------------------+

  | -(-9223372036854775808) |

  +-------------------------+

  |               854775808 |

  +-------------------------+

Which is obviously wrong.