Don't skip past empty common prefixes
Don't skip past empty common prefixes

Fixes #654. If the list of so-far-matching common prefixes is empty, we
shouldn't try and skip past the last (sorted) common prefix. Otherwise,
we try and call `lists:last/1` on the empty list.

sdebnath/riak_cs,dragonfax/riak_cs,laurenrother/riak_cs,basho/riak_cs,basho/riak_cs,laurenrother/riak_cs,sdebnath/riak_cs,GabrielNicolasAvellaneda/riak_cs,GabrielNicolasAvellaneda/riak_cs,sdebnath/riak_cs,laurenrother/riak_cs,GabrielNicolasAvellaneda/riak_cs,yangchengjian/riak_cs,sdebnath/riak_cs,basho/riak_cs,GabrielNicolasAvellaneda/riak_cs,yangchengjian/riak_cs,laurenrother/riak_cs,dragonfax/riak_cs,yangchengjian/riak_cs,dragonfax/riak_cs,yangchengjian/riak_cs,basho/riak_cs,dragonfax/riak_cs