Fix tag update when calculation fail.

When tag calculation fails and the same tag is stopped as a side effect, we
don't try to update its value anymore.

ccnode/tags.py

@@ -67,6 +67,13 @@ class Tag(object):
         prev_value = self.value
         self.calculate_value()
 
+        if self.db is None:
+            # when we calculate the tag, the latter could raise an exception and
+            # could provoke a deconnection to the libvirt for example, that will
+            # also provoke a deregister of some tags which might include the
+            # current (in case this happens, just return)
+            return
+
         if prev_value == self.value:
             return
         elif prev_value is None: