admin: fix possible crash when monitoring a backup cluster that has just switch to BACKINGUP state

This fixes:

  Traceback (most recent call last):
    ...
    File "neo/admin/handler.py", line 200, in answerLastTransaction
      app.maybeNotify(name)
    File "neo/admin/app.py", line 380, in maybeNotify
      self._notify(False)
    File "neo/admin/app.py", line 302, in _notify
      body += '', name, '    ' + backup.formatSummary(upstream)[1]
    File "neo/admin/app.py", line 74, in formatSummary
      tid = self.backup_tid if backup else self.ltid
  AttributeError: 'Backup' object has no attribute 'backup_tid'

neo/admin/app.py

@@ -69,11 +69,21 @@ class Monitor(object):
             ) if summary else str(self.cluster_state)
         if self.down:
             summary += '; DOWN=%s' % self.down
-        if self.operational:
-            backup = self.cluster_state == ClusterStates.BACKINGUP
-            tid = self.backup_tid if backup else self.ltid
+        while self.operational: # not a loop
+            if self.cluster_state == ClusterStates.BACKINGUP:
+                try:
+                    tid = self.backup_tid
+                except AttributeError:
+                    # Possible race when a backup cluster has just switched to
+                    # BACKINGUP state, whereas the next call to app._notify
+                    # will be with ask_ids=False, which means that backup_tid
+                    # will be unknown for this cluster.
+                    break
+            else:
+                tid = self.ltid
+                upstream = None
             x = datetimeFromTID(tid)
-            if upstream and backup:
+            if upstream:
                 lag = (upstream[0] - x).total_seconds()
                 if lag or tid >= upstream[1]:
                     lagging = self.max_lag < lag