Spotted while testing https://gerrit.wikimedia.org/r/#/c/431485/
Stacktrace can be found at https://integration.wikimedia.org/ci/job/quibble-vendor-mysql-hhvm-docker/1227/console and error details copied below as follows:
```
23:16:55 1) MergeUserTest::testDeleteUser
23:16:55 Wikimedia\Rdbms\DBQueryError: A database query error has occurred. Did you forget to run your application's database schema updater after upgrading?
23:16:55 Query: UPDATE `unittest_site_stats` SET ss_users=ss_users-1
23:16:55 Function: SiteStatsUpdate::tryDBUpdateInternal
23:16:55 Error: 1690 BIGINT UNSIGNED value is out of range in '(`wikidb`.`unittest_site_stats`.`ss_users` - 1)' (/tmp/quibble-mysql-5gyps4x0/socket)
23:16:55
23:16:55
23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:1449
23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:1419
23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:1192
23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:2060
23:16:55 /workspace/src/includes/deferred/SiteStatsUpdate.php:132
23:16:55 /workspace/src/includes/deferred/MWCallableUpdate.php:34
23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:268
23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:226
23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:134
23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:307
23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:100
23:16:55 /workspace/src/extensions/UserMerge/MergeUser.php:514
23:16:55 /workspace/src/extensions/UserMerge/MergeUser.php:61
23:16:55 /workspace/src/extensions/UserMerge/tests/phpunit/MergeUserTest.php:73
23:16:55 /workspace/src/tests/phpunit/MediaWikiTestCase.php:423
23:16:55 /workspace/src/maintenance/doMaintenance.php:94
```