Fix negative stat value issue after attempted sync

-Better error handling of update_db()
-Fixes an issue that can cause some stats values to get set to a negative number when running an index sync and the wallet daemon has not finished loading the blockchain yet (If this happens to you, stop the explorer, apply this fix with `git pull` to make sure it doesn't happen again, then run a reindex-txcount and reindex-last to restore the proper values)

lib/database.js

@@ -828,7 +828,8 @@ module.exports = {
   // updates stats data for given coin; called by sync.js
   update_db: function(coin, cb) {
     lib.get_blockcount( function (count) {
-      if (!count) {
+      // check to ensure count is a positive number
+      if (!count || (count != null && typeof count === 'number' && count < 0)) {
         console.log('Unable to connect to explorer API');
         return cb(false);
       }