channel [nfc]: Guard ChannelDeleteEvent against unexpected sever behavior

See the motivation behind this:
  zulip#880 (comment)

Author: sm-sayedi

lib/model/channel.dart

@@ -192,12 +192,19 @@ class ChannelStoreImpl with ChannelStore {
 
       case ChannelDeleteEvent():
         for (final stream in event.streams) {
-          assert(identical(streams[stream.streamId], streamsByName[stream.name]));
-          assert(subscriptions[stream.streamId] == null
-            || identical(subscriptions[stream.streamId], streams[stream.streamId]));
-          streams.remove(stream.streamId);
-          streamsByName.remove(stream.name);
-          subscriptions.remove(stream.streamId);
+          final localStream = streams[stream.streamId];
+          if (localStream == null) continue; // TODO(log)
+
+          final ZulipStream(:streamId, name: String streamName) = localStream;
+          if (streamName != stream.name) {
+            // TODO(log)
+          }
+          assert(identical(streams[streamId], streamsByName[streamName]));
+          assert(subscriptions[streamId] == null
+            || identical(subscriptions[streamId], streams[streamId]));
+          streams.remove(streamId);
+          streamsByName.remove(streamName);
+          subscriptions.remove(streamId);
         }
     }
   }