fix hang after previous request failed

lib/directoryindex.js

@@ -40,14 +40,12 @@ DirectoryIndex.prototype.update = function() {
     var index = this;
     fs.readdir(index.filepath, function(err, files) {
         index.err = err;
-        if (!err) {
-            for (var result = index.list = {}, i = 0; i < files.length; i++) {
-                var name = files[i].match(/^([\w-]+)\.mbtiles$/);
-                if (name) {
-                    result[name[1]] = 'mbtiles://' + path.join(index.filepath, name[0]);
-                }
+        index.list = {};
+        if (!err) for (var i = 0; i < files.length; i++) {
+            var name = files[i].match(/^([\w-]+)\.mbtiles$/);
+            if (name) {
+                index.list[name[1]] = 'mbtiles://' + path.join(index.filepath, name[0]);
             }
-            index.list = result;
         }
         index.emit('updated');
     });

test/list.test.js

@@ -0,0 +1,29 @@
+process.env.NODE_ENV = 'test';
+
+var fs = require('fs');
+var Step = require('step');
+var assert = require('assert');
+var MBTiles = require('..');
+
+var fixtures = {
+    doesnotexist: __dirname + '/doesnotexist'
+};
+
+try { fs.unlink(fixtures.doesnotexist); } catch (err) {}
+
+
+exports['list'] = function(beforeExit) {
+    var completed = false; beforeExit(function() { assert.ok(completed); });
+
+    MBTiles.list(fixtures.doesnotexist, function(err, list) {
+        assert.ok(err);
+        assert.equal(err.message, 'ENOENT, No such file or directory');
+        
+        MBTiles.list(fixtures.doesnotexist, function(err, list) {
+            completed = true;
+            assert.ok(err);
+            assert.equal(err.message, 'ENOENT, No such file or directory');
+        });
+    });
+};
+