Handle SSL_shutdown while in init more appropriately

Calling SSL_shutdown while in init previously gave a "1" response, meaning
everything was successfully closed down (even though it wasn't). Better is
to send our close_notify, but fail when trying to receive one.

The problem with doing a shutdown while in the middle of a handshake is
that once our close_notify is sent we shouldn't really do anything else
(including process handshake/CCS messages) until we've received a
close_notify back from the peer. However the peer might send a CCS before
acting on our close_notify - so we won't be able to read it because we're
not acting on CCS messages!

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>

ssl/ssl_lib.c

@@ -1564,10 +1564,7 @@ int SSL_shutdown(SSL *s)
         return -1;
     }
 
-    if (!SSL_in_init(s))
-        return (s->method->ssl_shutdown(s));
-    else
-        return (1);
+    return s->method->ssl_shutdown(s);
 }
 
 int SSL_renegotiate(SSL *s)