qvm-sync-clock: Do not fail if clockvm is not set

qvm-tools/qvm-sync-clock

@@ -32,6 +32,9 @@ def main():
     app = Qubes()
     clockvm = app.clockvm
 
+    if not clockvm:
+        sys.exit(0)
+
     if not clockvm.is_running():
         sys.stderr.write('ClockVM {} is not running, aborting.\n'.format(
             clockvm.name))