Couple of fixes for initial indexing - stats.supply and stats.last

lib/database.js

@@ -746,7 +746,7 @@ module.exports = {
               count : count,
               supply: supply,
               connections: connections,
-              last: new_stats.last});
+              last: (new_stats.last ? new_stats.last : 0)});
           });
         });
       });

lib/explorer.js

@@ -15,7 +15,7 @@ function coinbase_supply(cb) {
     if (address) {
       return cb(address.sent);
     } else {
-      return cb();
+      return cb(0);
     }
   });
 }