User.verify_code throws exception when user does not exist

[adamrp]

these lines select the verification code that's stored in the database for the user so that it can be compared with the verification code supplied by the user on a password reset or user creation. Line 275 assumes that the user exists and throws an exception if it does not; we are seeing this error on the server:

Traceback (most recent call last):

  File "/home/qiita/.virtualenvs/qiita/lib/python2.7/site-packages/tornado/web.py", line 1141, in _when_complete
    callback()

  File "/home/qiita/.virtualenvs/qiita/lib/python2.7/site-packages/tornado/web.py", line 1162, in _execute_method
    self._when_complete(method(*self.path_args, **self.path_kwargs),

  File "/home/qiita/qiita_main/qiita_pet/handlers/auth_handlers.py", line 66, in get
    if User.verify_code(email, code, "create"):

  File "/home/qiita/qiita_main/qiita_db/user.py", line 275, in verify_code
    db_code = conn_handler.execute_fetchone(sql, (email,))[0]

TypeError: 'NoneType' object has no attribute '__getitem__'

We should check whether the execute_fetchone actually returned anything before trying to access index 0.