A protocol byte is converted to enum MessageKind. What should happen before using it to i…

A protocol byte is converted to enum MessageKind. What should happen before using it to index a handler table?

1. Validate that it equals one of the supported enumerator values
2. Assume the conversion clamps it to the nearest enumerator
3. Check only that sizeof(MessageKind) equals one
4. Read it through a union to make it type-safe