Set flags to 0 before calling BN_with_flags()

BN_with_flags() will read the dest->flags to keep the BN_FLG_MALLOCED but
overwrites everything else.
Signed-off-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NRich Salz <rsalz@openssl.org>
MR #1231

crypto/bn/bn_gcd.c

@@ -599,6 +599,7 @@ static BIGNUM *BN_mod_inverse_no_branch(BIGNUM *in,
          * BN_div_no_branch will be called eventually.
          */
         pB = &local_B;
+        local_B.flags = 0;
         BN_with_flags(pB, B, BN_FLG_CONSTTIME);
         if (!BN_nnmod(B, pB, A, ctx))
             goto err;
@@ -626,6 +627,7 @@ static BIGNUM *BN_mod_inverse_no_branch(BIGNUM *in,
          * BN_div_no_branch will be called eventually.
          */
         pA = &local_A;
+        local_A.flags = 0;
         BN_with_flags(pA, A, BN_FLG_CONSTTIME);
 
         /* (D, M) := (A/B, A%B) ... */