[3.12] gh-117310: Remove extra DECREF on "no ciphers" error path in _ssl._SSLContext constructor (GH-117309) (GH-117317)

gh-117310: Remove extra DECREF on "no ciphers" error path in `_ssl._SSLContext` constructor (GH-117309)

Remove extra self DECREF on ssl "no ciphers" error path.

This doesn't come up in practice because nobody links against a broken
OpenSSL library that provides nothing.
(cherry picked from commit 8cb7d7f)

Co-authored-by: Gregory P. Smith <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Library/2024-03-27-21-05-52.gh-issue-117310.Bt2wox.rst

@@ -0,0 +1,4 @@
+Fixed an unlikely early & extra ``Py_DECREF`` triggered crash in :mod:`ssl`
+when creating a new ``_ssl._SSLContext`` if CPython was built implausibly such
+that the default cipher list is empty **or** the SSL library it was linked
+against reports a failure from its C ``SSL_CTX_set_cipher_list()`` API.

Modules/_ssl.c

@@ -3143,7 +3143,6 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
         result = SSL_CTX_set_cipher_list(ctx, "HIGH:!aNULL:!eNULL");
     }
     if (result == 0) {
-        Py_DECREF(self);
         ERR_clear_error();
         PyErr_SetString(get_state_ctx(self)->PySSLErrorObject,
                         "No cipher can be selected.");