fix: throw error when encoding/decoding PONG with invalid port

[jtraglia]

According to the spec, recipient-port is an (unsigned) 16-bit integer. This PR ensures that those port values are valid.

• Remove a commented out line of code, doesn't seem useful to keep.
• When decoding, check that the length of the field isn't greater than 2 bytes.
• When encoding, check that the number is in the valid port range.
• Add a test for encoding a PONG message with a 1-byte port.
• Add a new test function that handles invalid messages.

[wemeetagain]

🙏