Return a fatal error if application data is encountered during shutdown

Currently if you encounter application data while waiting for a
close_notify from the peer, and you have called SSL_shutdown() then
you will get a -1 return (fatal error) and SSL_ERROR_SYSCALL from
SSL_get_error(). This isn't accurate (it should be SSL_ERROR_SSL) and
isn't persistent (you can call SSL_shutdown() again and it might then work).

We change this into a proper fatal error that is persistent.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6340)

ssl/ssl_err.c

diff --git a/ssl/ssl_err.c b/ssl/ssl_err.c
index 03c5bf2..9ce643a 100644
--- a/ssl/ssl_err.c
+++ b/ssl/ssl_err.c
@@ -726,6 +726,8 @@
 };
 
 static const ERR_STRING_DATA SSL_str_reasons[] = {
+    {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_APPLICATION_DATA_AFTER_CLOSE_NOTIFY),
+    "application data after close notify"},
     {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_APP_DATA_IN_HANDSHAKE),
     "app data in handshake"},
     {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_ATTEMPT_TO_REUSE_SESSION_IN_DIFFERENT_CONTEXT),