Don't attempt to add a zero length padding extension

The padding extension should always be at least 1 byte long
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/4331)

ssl/statem/extensions_clnt.c

@@ -843,7 +843,7 @@ EXT_RETURN tls_construct_ctos_padding(SSL *s, WPACKET *pkt,
          * 1 byte long so as not to have an empty extension last (WebSphere 7.x,
          * 8.x are intolerant of that condition)
          */
-        if (hlen >= 4)
+        if (hlen > 4)
             hlen -= 4;
         else
             hlen = 1;