fix userid conversion

src/Discord.Net.Core/Utils/TokenUtils.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Text;
 
 namespace Discord
 {
@@ -41,10 +42,10 @@ namespace Discord
             try
             {
                 // decode the first segment as base64
-                var v = Convert.FromBase64String(segments[0]);
-                BitConverter.ToUInt64(v, 0);
-                // if no exception thrown, token is valid
-                return true;
+                var bytes = Convert.FromBase64String(segments[0]);
+                var idStr = Encoding.UTF8.GetString(bytes);
+                // discard id
+                return ulong.TryParse(idStr, out var id);
             }
             catch (FormatException)
             {

test/Discord.Net.Tests/Tests.TokenUtils.cs

@@ -138,6 +138,7 @@ namespace Discord
         [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4. this part is invalid. this part is also invalid", true)]
         [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.", false)]
         [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4", false)]
+        [InlineData("NDI4NDc3OTQ0MDA5MTk1NTIw.xxxx.xxxxx", true)]
         // should not throw an unexpected exception
         [InlineData("", false)]
         [InlineData(null, false)]