modes/ocb128.c: improve the calculation of double mask

CLA: trivial
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6667)

crypto/modes/ocb128.c

@@ -74,7 +74,7 @@ static void ocb_double(OCB_BLOCK *in, OCB_BLOCK *out)
      */
     mask = in->c[0] & 0x80;
     mask >>= 7;
-    mask *= 135;
+    mask = (0 - mask) & 0x87;
 
     ocb_block_lshift(in->c, 1, out->c);