Fix seqno_minus for negative values.
Fix seqno_minus for negative values.

Drooids/babeld,Gwendocg/babeldToS,tcatm/babeld,wlanslovenija/babeld,jech/babeld,sudomesh/babeld,woniullb/babeld,jech/babeld,dtaht/babeld-shortrtt-metrics,Gwendocg/babeldToS,boutier/babeld,wlanslovenija/babeld