Write unit tests for message parsing code

[jmcph4]

Currently, the message parsing code (i.e. the conversion methods to_bytes and from_bytes for the Message trait) present the largest attack surface against tftpd (and the client also).

Unit tests are required for all message parsing code, covering the following test cases:

RRQ

• Mismatched opcode
• Missing filename
• Unterminated filename
• Missing mode string
• Unterminated mode string
• Unterminated filename and unterminated mode string
• Too few bytes
• Too many bytes

WRQ

• Mismatched opcode
• Missing filename
• Unterminated filename
• Missing mode string
• Unterminated mode string
• Unterminated filename and unterminated mode string
• Too few bytes
• Too many bytes

DATA

• Mismatched opcode
• No data
• Too few bytes
• Too many bytes

ACK

• Mismatched opcode
• Too few bytes
• Too many bytes

ERROR

• Mismatched opcode
• Missing error message
• Unterminated error message
• Too few bytes
• Too many bytes