Assert that SSLfatal() only gets called once

We shouldn't call SSLfatal() multiple times for the same error condition.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4778)
diff --git a/ssl/statem/statem_clnt.c b/ssl/statem/statem_clnt.c
index 4bd9457..038fac9 100644
--- a/ssl/statem/statem_clnt.c
+++ b/ssl/statem/statem_clnt.c
@@ -2938,10 +2938,12 @@
     return 1;
  err:
     EVP_PKEY_free(ckey);
-#endif
+    return 0;
+#else
     SSLfatal(s, SSL_AD_INTERNAL_ERROR, SSL_F_TLS_CONSTRUCT_CKE_DHE,
              ERR_R_INTERNAL_ERROR);
     return 0;
+#endif
 }
 
 static int tls_construct_cke_ecdhe(SSL *s, WPACKET *pkt)