Ensure we raise SSLfatal on error

We were missing a call to SSLfatal. A comment claimed that we had already
called it - but that is incorrect.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/13229)

ssl/statem/statem_srvr.c

@@ -2588,7 +2588,7 @@ int tls_construct_server_key_exchange(SSL *s, WPACKET *pkt)
 
         s->s3.tmp.pkey = ssl_generate_pkey(s, pkdhp);
         if (s->s3.tmp.pkey == NULL) {
-            /* SSLfatal() already called */
+            SSLfatal(s, SSL_AD_INTERNAL_ERROR, 0, ERR_R_INTERNAL_ERROR);
             goto err;
         }