Clean up the check to make sure we can get the next header field.

Clean up the check to make sure we can get the next header field.

Just check whether the pointer to it is within the available packet data
- and, if it's not, report truncation.

While we're at it:

Make the initialization vector length unsigned; it's either zero or a
positive number.

Rename a variable used for the ciphertext len to ctlen.