Fix divided by zero issue in case of words are not found in dict.

碰巧在处理大量文本时抓到这个bug

错误重现方法：
import jieba.analyse.textrank as rk
rk('阿修罗闪空')

在输入文本较短而没有找到任何录入词典的生词时
会出现divided by zero错误