qmemman: fix null reference bug

qmemman/qmemman_server.py

@@ -74,6 +74,10 @@ class XS_Watcher:
         self.log.debug('domain_list_changed(param={!r})'.format(param))
 
         curr = self.handle.ls('', '/local/domain')
+
+        if curr == None:
+            return
+
         # check if domain is really there, it may happen that some empty
         # directories are left in xenstore
         curr = filter(
@@ -86,9 +90,6 @@ class XS_Watcher:
 
         self.log.debug('curr={!r}'.format(curr))
 
-        if curr == None:
-            return
-
         self.log.debug('acquiring global_lock')
         global_lock.acquire()
         self.log.debug('global_lock acquired')