prevent fixLinks error when href is NoneType object

This commit prevent href is NonType object when execute fixLinks function, the error info like following:

fixing links in  /media/build/Workspace/blog/static/shi-yong-shairport-rang-ni-de-raspberry-pi-bian-cheng-airplay-yin-le-bo-fang-qi/index.html
	../index.html => ../
	../rss/index.html => ../rss/index.rss
Traceback (most recent call last):
  File "/home/coldnew/.local/bin/buster", line 9, in <module>
    load_entry_point('buster==0.1.3', 'console_scripts', 'buster')()
  File "/home/coldnew/.local/lib64/python2.7/site-packages/buster/buster.py", line 91, in main
    newtext = fixLinks(filetext, parser)
  File "/home/coldnew/.local/lib64/python2.7/site-packages/buster/buster.py", line 69, in fixLinks
    if not abs_url_regex.search(href):
TypeError: expected string or buffer