Label length check using 16-bit instead of 8-bit

[martoko]

This should likely check the UTF-8 encoded version of the string or something similar, not the default C# representation of string. Which is, at least on windows, a UTF-16 encoded string.

https://github.com/richardschneider/net-dns/blob/701463d2091e6d98d4cc4490abb0e0ead8ae2985/src/WireWriter.cs#L273