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



diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 5806417..a8c339d 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -4545,11 +4545,12 @@
          * 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);