Fix s_client hang on non-compliant NNTP server test

Fixes #7722

CLA: trivial

Signed-off-by: Mateja Milosevic <quantumgleam@gmail.com>

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7915)