Check if we actually have a database.

Fixes segfault when running akabei repo
Workaround until we have a proper fix

Reviewed at https://chakraos.org/reviewboard/r/61/

syncoperation.cpp

@@ -311,7 +311,7 @@ void SyncOperation::listRepo(const QString &repo)
             break;
         }
     }
-    if (!db) {
+    if (!db || !db->database()) {
         QTextStream err(stderr);
         err << Akabei::errorPrefix << QObject::tr("No repository found!") << endl;
         nextOperation();