Ensure we fail with a decode error alert if the server sends and empty Cert

Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3436)

ssl/statem/statem_clnt.c

@@ -1688,7 +1688,8 @@ MSG_PROCESS_RETURN tls_process_server_certificate(SSL *s, PACKET *pkt)
     if ((SSL_IS_TLS13(s) && !PACKET_get_1(pkt, &context))
             || context != 0
             || !PACKET_get_net_3(pkt, &cert_list_len)
-            || PACKET_remaining(pkt) != cert_list_len) {
+            || PACKET_remaining(pkt) != cert_list_len
+            || PACKET_remaining(pkt) == 0) {
         al = SSL_AD_DECODE_ERROR;
         SSLerr(SSL_F_TLS_PROCESS_SERVER_CERTIFICATE, SSL_R_LENGTH_MISMATCH);
         goto f_err;