NullRef fix

src/Discord.Net.Commands/Extensions/MessageExtensions.cs

@@ -45,7 +45,7 @@ namespace Discord.Commands
         public static bool HasMentionPrefix(this IUserMessage msg, IUser user, ref int argPos)
         {
             var text = msg.Content;
-            if (text.Length <= 3 || text[0] != '<' || text[1] != '@') return false;
+            if (string.IsNullOrEmpty(text) || text.Length <= 3 || text[0] != '<' || text[1] != '@') return false;
 
             int endPos = text.IndexOf('>');
             if (endPos == -1) return false;