ssl/statem/extensions_clnt.c: fix return code buglet.

Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4297)

ssl/statem/extensions_clnt.c

@@ -1052,7 +1052,7 @@ EXT_RETURN tls_construct_ctos_psk(SSL *s, WPACKET *pkt, unsigned int context,
  err:
     return ret;
 #else
-    return EXT_RETURN_SENT;
+    return EXT_RETURN_NOT_SENT;
 #endif
 }