[BUG] `hexo server` hanging forever when repo contains any invalid symbolic link

## Environment Info

Node version(`node -v`): v6.9.1 (in `nvm`)
OS: Chakra Linux

Your site `_config.yml` (Optional):

Your theme `_config.yml` (Optional):

Hexo and Plugin version(`npm ls --depth 0`):
```
hexo-site@0.0.0 /home/onohiroko/source-blog
├── hexo@3.2.2
├── hexo-cli@1.0.4
├── hexo-deployer-git@0.0.3
├── hexo-generator-archive@0.1.4
├── hexo-generator-category@0.1.3
├── hexo-generator-feed@1.2.0
├── hexo-generator-index@0.1.3
├── hexo-generator-sitemap@1.1.2
├── hexo-generator-tag@0.1.2
├── hexo-renderer-ejs@0.1.1
├── hexo-renderer-marked@0.2.11
├── hexo-renderer-stylus@0.2.3
├── hexo-server@0.1.3
└── xml2js@0.4.17
```

## For question
After `hexo server --debug`, it stopped at a fixed point, then hang and server still not startup:

```sh
./node_modules/hexo-cli/bin/hexo server --debug (01-04 10:11)
02:11:19.915 DEBUG Hexo version: 3.2.2
02:11:19.917 DEBUG Working directory: ~/source-blog/
02:11:19.988 DEBUG Config loaded: ~/source-blog/_config.yml
02:11:20.101 DEBUG Script loaded: scripts/livedoor-opml2md.js
02:11:20.101 DEBUG Script loaded: scripts/toc.js
02:11:20.102 DEBUG Script loaded: themes/taiga/scripts/fancybox.js
02:11:20.103 DEBUG Plugin loaded: hexo-cli
02:11:20.105 DEBUG Plugin loaded: hexo-generator-archive
02:11:20.106 DEBUG Plugin loaded: hexo-generator-category
02:11:20.144 DEBUG Plugin loaded: hexo-deployer-git
02:11:20.146 DEBUG Plugin loaded: hexo-generator-feed
02:11:20.146 DEBUG Plugin loaded: hexo-generator-index
02:11:20.147 DEBUG Plugin loaded: hexo-generator-sitemap
02:11:20.149 DEBUG Plugin loaded: hexo-renderer-ejs
02:11:20.230 DEBUG Plugin loaded: hexo-renderer-stylus
02:11:20.231 DEBUG Plugin loaded: hexo-generator-tag
02:11:20.235 DEBUG Plugin loaded: hexo-renderer-marked
02:11:20.273 DEBUG Plugin loaded: hexo-server
02:11:20.280 DEBUG Loading database.
02:11:20.451 INFO Start processing
02:11:20.654 DEBUG Processed: _drafts/2017-malaysia-singapore.md
......... (articles)
02:11:20.721 DEBUG Theme config loaded.
02:11:20.722 DEBUG Processed: _config.yml
02:11:20.722 DEBUG Processed: image/banner/03.jpg
02:11:20.722 DEBUG Processed: image/banner/02.jpg
02:11:20.723 DEBUG Processed: lists/dist/ui-bootstrap-tpls-2.5.0.min.js
02:11:20.723 DEBUG Processed: transcription/Skyrim_Main_Theme.pdf
......... (files)
02:11:20.854 DEBUG Processed: image/2014/IMG_6379.jpg
02:11:20.854 DEBUG Processed: image/2014/IMG_6382.jpg
02:11:20.860 DEBUG Processed: image/2014/IMG_6386.jpg
02:11:20.897 DEBUG Processed: file/2015/1947_Overture_cover.pdf
^C02:14:02.583 INFO See you again
02:14:02.633 DEBUG Database saved
```
It freezed at `02:11:20.897`, so I press `^C` to interrupt.

Some possibly usable clues:
1. Firewall is not running.
2. I didn't encounter this problem on macOS.
3. `4000` port is not be occupied by others.
4. `hexo` is not installed system globally.