NTP: Improve length checks.

NTP: Improve length checks.

In ntp_print() add a missing length check to reject packets that are
declared too short, make the existing length checks easier to follow and
add a catch-all block after the known message layouts.

This fixes a bug where an invalid packet could be erroneously printed
like it is valid so long as the provided buffer was large enough. That
said, the bounds checks were done correctly so there was no over-read.