Add a missing null-conditional in CommandContext#ctor

Prevents an NRE when creating a CommandContext from a message in an IDMChannel

src/Discord.Net.Commands/CommandContext.cs

@@ -21,7 +21,7 @@
         public CommandContext(IDiscordClient client, IUserMessage msg)
         {
             Client = client;
-            Guild = (msg.Channel as IGuildChannel).Guild;
+            Guild = (msg.Channel as IGuildChannel)?.Guild;
             Channel = msg.Channel;
             User = msg.Author;
             Message = msg;