Config.__del__() can raise exception

[MarcelWaldvogel]

Trying to work around #915, I started writing code that would try to call Config.get_*_config() to check whether they are out there.

However, if Config.__init__() raises an exception (e.g., because the .gitconfig file is not readable or is a directory), then on program termination (or garbage collection), a seemingly spurious exception is raised:

Exception ignored in: <function Config.__del__ at 0x7fe011d96730>
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pygit2/config.py", line 99, in __del__
    C.git_config_free(self._config)
AttributeError: 'Config' object has no attribute '_config'

This happens also if the exception is caught, i.e., when the exception from __init__() is not seen by the user.