Don't crash when user/channel mentions are received in a PM

src/Discord.Net/Helpers/Mention.cs

@@ -24,21 +24,24 @@ namespace Discord
 			text = _userRegex.Replace(text, new MatchEvaluator(e =>
 			{
 				string id = e.Value.Substring(2, e.Value.Length - 3);
-				var user = client.Users[id, server.Id];
+				var user = client.Users[id, server?.Id];
 				if (user != null)
 					return '@' + user.Name;
 				else //User not found
 					return '@' + e.Value;
 			}));
-			text = _channelRegex.Replace(text, new MatchEvaluator(e =>
+			if (server != null)
 			{
-				string id = e.Value.Substring(2, e.Value.Length - 3);
-				var channel = client.Channels[id];
-				if (channel != null && channel.Server.Id == server.Id)
-					return '#' + channel.Name;
-				else //Channel not found
+				text = _channelRegex.Replace(text, new MatchEvaluator(e =>
+				{
+					string id = e.Value.Substring(2, e.Value.Length - 3);
+					var channel = client.Channels[id];
+					if (channel != null && channel.Server.Id == server.Id)
+						return '#' + channel.Name;
+					else //Channel not found
 					return '#' + e.Value;
-			}));
+				}));
+			}
 			return text;
 		}