bpo-41252: Fix incorrect refcounting in _ssl.c's _servername_callback() (GH-21407)

(cherry picked from commit ee96f32)

Co-authored-by: Zackery Spytz <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2020-07-08-21-55-23.bpo-41252.nBWL-Y.rst

@@ -0,0 +1 @@
+Fix incorrect refcounting in _ssl.c's ``_servername_callback()``.

Modules/_ssl.c

@@ -4545,11 +4545,12 @@ _servername_callback(SSL *s, int *al, void *args)
          * back into a str object, but still as an A-label (bpo-28414)
          */
         servername_str = PyUnicode_FromEncodedObject(servername_bytes, "ascii", NULL);
-        Py_DECREF(servername_bytes);
         if (servername_str == NULL) {
             PyErr_WriteUnraisable(servername_bytes);
+            Py_DECREF(servername_bytes);
             goto error;
         }
+        Py_DECREF(servername_bytes);
         result = PyObject_CallFunctionObjArgs(
             ssl_ctx->set_sni_cb, ssl_socket, servername_str,
             ssl_ctx, NULL);