store: fix length check in uniqueNameURLs
store: fix length check in uniqueNameURLs

I mentioned this in the review, but I think it must
have been obscured by another suggestion I made
in the same comment.

R=fwereade, niemeyer
CC=
https://codereview.appspot.com/5720051

juju/charmstore,mhilton/juju-charmstore,urosj/charmstore-1,urosj/charmstore-1,ericsnowcurrently/juju-charmstore,fabricematrat/charmstore,frankban/charmstore,juju/charmstore,mhilton/charmstore,ericsnowcurrently/juju-charmstore,urosj/charmstore-1,fabricematrat/charmstore,makyo/charmstore,wallyworld/charmstore,jrwren/charmstore,fabricematrat/charmstore,wallyworld/charmstore,makyo/charmstore,frankban/charmstore,ericsnowcurrently/juju-charmstore,juju/charmstore,mhilton/charmstore,jrwren/charmstore,mhilton/juju-charmstore,mhilton/juju-charmstore,wallyworld/charmstore,jrwren/charmstore,mhilton/charmstore,makyo/charmstore,frankban/charmstore