fix: def_list.py raises exception for unintended : in list element.

Markdown source

~~~~

- hoge
-    :    hogehoge

~~~~
 
will raise exception like this:

 File "env/lib/python2.7/site-packages/markdown/extensions/def_list.py", line 51, in run
    if not terms and sibling.tag == 'p':

  'NoneType' object has no attribute 'tag'