Add graceful error handling for less than 2 bytes packet

[mayurikini]

When request for a single byte is made it panics (see below). Adding code to gracefully error out when a single byte is sent, as it doesn’t align with standard tftp protocol.

thread 'main' panicked at src/packet.rs:61:59:
range end index 2 out of range for slice of length 1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace