IndexError with new trailing hyphen check #82
Description
#64 introduced the following uncaught exception. Is this intended?
>>> idna.ulabel('xn--')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/John/.pyenv/versions/3.6.5/Python.framework/Versions/3.6/lib/python3.6/site-packages/idna/core.py", line 303, in ulabel
if label.decode('ascii')[-1] == '-':
IndexError: string index out of rangeI would've expected UnicodeError or IDNAError, which are more descriptive.