fix x86_64-specific crash with one-word modulus.

PR: #3397

(Imported from upstream's 47b9e06cfd)

Change-Id: I92d46a3132233c179f4b708d506bfb7212c26a33

crypto/bn/exponentiation.c

@@ -982,7 +982,7 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 
   /* Dedicated window==4 case improves 512-bit RSA sign by ~15%, but as
    * 512-bit RSA is hardly relevant, we omit it to spare size... */
-  if (window == 5) {
+  if (window == 5 && top > 1) {
     void bn_mul_mont_gather5(BN_ULONG * rp, const BN_ULONG * ap,
                              const void * table, const BN_ULONG * np,
                              const BN_ULONG * n0, int num, int power);