Avoid using invalid iterator in LLNotifications::getChannel if channel is not found.

indra/llui/llnotifications.cpp

@@ -1094,6 +1094,7 @@ LLNotificationChannelPtr LLNotifications::getChannel(const std::string& channelN
 	if(p == mChannels.end())
 	{
 		llerrs << "Did not find channel named " << channelName << llendl;
+		return LLNotificationChannelPtr();
 	}
 	return p->second;
 }