py: fix exception during cleanup of half-initialized Nftables

py: fix exception during cleanup of half-initialized Nftables

When we create a Nftables instance against an older library version,
we might not find a symbol and fail with an exception when initializing
the context object.

Then, __del__() is still called, but resulting in a second exception
because self.__ctx is not set. Avoid that second exception.

    $ python -c 'import nftables; nftables.Nftables()'
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/data/src/nftables/py/nftables.py", line 90, in __init__
        self.nft_ctx_input_get_flags = lib.nft_ctx_input_get_flags
                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/ctypes/__init__.py", line 389, in __getattr__
        func = self.__getitem__(name)
               ^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/ctypes/__init__.py", line 394, in __getitem__
        func = self._FuncPtr((name_or_ordinal, self))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    AttributeError: /lib64/libnftables.so.1: undefined symbol: nft_ctx_input_get_flags
    Exception ignored in: <function Nftables.__del__ at 0x7f6315a2c540>
    Traceback (most recent call last):
      File "/data/src/nftables/py/nftables.py", line 166, in __del__
        self.nft_ctx_free(self.__ctx)
        ^^^^^^^^^^^^^^^^^
    AttributeError: 'Nftables' object has no attribute 'nft_ctx_free'

Signed-off-by: Thomas Haller <thaller@redhat.com>
Reviewed-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>