Merge pull request #1685 from MessagePack-CSharp/fix1586

Remove `long` to `int` truncation of stream position

src/MessagePack.UnityClient/Assets/Scripts/MessagePack/MessagePackSerializer.cs

@@ -440,7 +440,7 @@ private static T DeserializeFromSequenceAndRewindStreamIfPossible<T>(Stream stre
             if (streamToRewind.CanSeek && !reader.End)
             {
                 // Reverse the stream as many bytes as we left unread.
-                int bytesNotRead = checked((int)reader.Sequence.Slice(reader.Position).Length);
+                long bytesNotRead = reader.Sequence.Slice(reader.Position).Length;
                 streamToRewind.Seek(-bytesNotRead, SeekOrigin.Current);
             }