AsyncQuery::startExec() segfaults after ConnectionManager::closeAll()

[misje]

Running startExec() on a AsyncQuery segfaults if ConnectionManager::closeAll() has been called.

# 0  0x0000000000736f5f in ?? ()
# 1  0x00007fff03a7718a in end_server () from /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
# 2  0x00007fff03a77338 in cli_safe_read () from /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
# 3  0x00007fff03a73a96 in cli_read_prepare_result () from /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
# 4  0x00007fff03a73e6d in mysql_stmt_prepare () from /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
# 5  0x00007fff223b928d in QMYSQLResult::prepare(QString const&) () from /usr/local/plugins/sqldrivers/libqsqlmysql.so
# 6  0x00007fff37141118 in QSqlResult::savePrepare(QString const&) () from /usr/local/lib/libQt5Sql.so.5
# 7  0x00007fff37131b15 in QSqlQuery::prepare(QString const&) () from /usr/local/lib/libQt5Sql.so.5
# 8  0x00007fff371cfbdb in Database::SqlTaskPrivate::run() () from […]