Deadlock when destroying server while a thread is in qDBusNewConnection

Noticed in a test failure: https://testresults.qt.io/logs/qt/qtbase/efd2577fdc2cdbddbddc688e2f1d0e2db0ba3d85/LinuxRHEL_7_4x86_64LinuxRHEL_7_4x86_64GCCqtci-linux-RHEL-7.4-x86_64-fa00a5Release_NoUseGoldLinker_ForceDebugInfo_SeparateDebugInfo/2a48a3b7dcb9652b1532a3ad14de7580b5b9b1ee/test_1553184371/log.txt.gz

The issue seems to occur when `void tst_QDBusConnection::connectToPeer()` fails on ` QVERIFY(con.isConnected());` after ` QDBusConnection::disconnectFromBus("bubu");`. That causes a 'return' and the server is starts destructing.

QDBusServer::~QDBusServer() locks `d->lock` followed by `QDBusConnectionManager::instance()->mutex`.

`static void qDBusNewConnection(DBusServer *server, DBusConnection *connection, void *data)` locks the same locks, but in the opposite order.