handle empty serial number table a bit better
handle empty serial number table a bit better

Before, this code would panic and cause various bits of code to
spin-loop. This way, we can at least figure out what's going on.

letsencrypt/boulder,ZCloud-Firstserver/boulder,patf/boulder,patf/boulder,bmw/boulder,ibukanov/boulder,kuba/boulder,mommel/boulder,r0ro/boulder,mommel/boulder,KyleChamberlin/boulder,KyleChamberlin/boulder,hlandauf/boulder,josephyzhou/boulder,jsha/boulder,postfix/boulder,kuba/boulder,kuba/boulder,postfix/boulder,lmcro/boulder,hlandauf/boulder,jsha/boulder,lmcro/boulder,lucasallan/boulder,benileo/boulder,postfix/boulder,josephyzhou/boulder,ibukanov/boulder,jcjones/boulder,modulexcite/boulder,mehmooda/boulder,josephyzhou/boulder,mommel/boulder,mehmooda/boulder,devpaul/boulder,mommel/boulder,mehmooda/boulder,letsencrypt/boulder,hlandauf/boulder,jsha/boulder,mehmooda/boulder,r0ro/boulder,kuba/boulder,deserted/boulder,ZCloud-Firstserver/boulder,jfrazelle/boulder,josephyzhou/boulder,devpaul/boulder,patf/boulder,jfrazelle/boulder,r0ro/boulder,lucasallan/boulder,r0ro/boulder,jcjones/boulder,deserted/boulder,KyleChamberlin/boulder,bmw/boulder,sophie-h/boulder,sophie-h/boulder,KyleChamberlin/boulder,mozmark/boulder,ibukanov/boulder,letsencrypt/boulder,postfix/boulder,jcjones/boulder,mozmark/boulder,devpaul/boulder,bmw/boulder,mozmark/boulder,sophie-h/boulder,modulexcite/boulder,lucasallan/boulder,modulexcite/boulder,deserted/boulder,jcjones/boulder,benileo/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,deserted/boulder,lmcro/boulder,mozmark/boulder,benileo/boulder,lmcro/boulder,modulexcite/boulder,benileo/boulder,patf/boulder,jfrazelle/boulder,hlandauf/boulder,ZCloud-Firstserver/boulder,devpaul/boulder