libalias: fix divide by zero causing panic

The packet_limit can fall to 0, leading to a divide by zero abort in
the "packets % packet_limit".

An possible solution would be to apply a lower limit of 1 after the
calculation of packet_limit, but since any number modulo 1 gives 0,
the more efficient solution is to skip the modulo operation for
packet_limit <= 1.

Since this is a fix for a panic observed in stable/12, merging this
fix to stable/12 and stable/13 before expiry of the 3 day waiting
period might be justified, if it works for the reporter of the issue.

Reported by: Karl Denninger <karl@denninger.net>
MFC after: 3 days