Better qualify bad packets

[erelson]

Currently we have the method is_bad_packet() which just evaluates if a packet is too short to contain the cmd/address byte. This might not even be valid, e.g. for status packets?

One case of bad packets I've seen is an 255 255 0 255 packet that showed up at the beginning of a log.

Other thoughts:

• Include check-sum checking in this method?
• Compare actual packet length (L+4) with the length byte (L)
• packet not long enough for at least one value byte = bad?
• does packet good/bad depend on the instruction byte?