testutils.c: Fix high bits of the mpz_urandomb used with mini-gmp.

ChangeLog

 2018-11-25  Niels Möller  <nisse@lysator.liu.se>
 
+	* testsuite/testutils.c (mpz_urandomb) [NETTLE_USE_MINI_GMP]: Fix
+	masking of most significant bits.
+
 	* rsa-decrypt-tr.c (rsa_decrypt_tr): Use
 	NETTLE_OCTET_SIZE_TO_LIMB_SIZE.
 

testsuite/testutils.c

@@ -818,7 +818,7 @@ mpz_urandomb (mpz_t r, struct knuth_lfib_ctx *ctx, mp_bitcnt_t bits)
   uint8_t *buf = xalloc (bytes);
 
   knuth_lfib_random (ctx, bytes, buf);
-  buf[bytes-1] &= 0xff >> (8*bytes - bits);
+  buf[0] &= 0xff >> (8*bytes - bits);
   nettle_mpz_set_str_256_u (r, bytes, buf);
   free (buf);
 }