Handle failure to call dlopen(NULL)

[indygreg]

See the inline comment for the explanation.

I have been running into this bug on PyOxidizer when using
Python distributions built against musl libc. For reference:

$ ldd python/install/bin/python3.7m
     not a dynamic executable

$ python/install/bin/python3.7m
Python 3.7.7 (default, Apr  5 2020, 06:02:52)
[Clang 9.0.1 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> ctypes.CDLL(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/gps/src/pyoxidizer.git/build/python_distributions/python.c1ffa330c730/python/install/lib/python3.7/ctypes/__init__.py", line 364, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: Dynamic loading not supported

Interestingly, OSError is incomplete (possibly a CPython ctypes bug?):

>>> try:
...     ctypes.CDLL(None)
... except OSError as e:
...     err = e
...
>>> err
OSError('Dynamic loading not supported')
>>> err.errno
None
>>> err.strerror
None
>>> err.args
('Dynamic loading not supported',)

[brettcannon]

Happy to take the fix, but we need a test to make sure we don't break this in the future.

[indygreg]

@brettcannon I believe this is ready for a re-review.