Don't let top go below zero!

diff --git a/crypto/bn/bn_mont.c b/crypto/bn/bn_mont.c
index e5fbcd619f7b05ce078565c7cdb67027916af7de..6a54e8f4cf843a67bbeaea6db0292c37113d7307 100644 (file)
--- a/crypto/bn/bn_mont.c
+++ b/crypto/bn/bn_mont.c
@@ -90,7 +90,7 @@ int BN_mod_mul_montgomery(BIGNUM *r, BIGNUM *a, BIGNUM *b,
                bn_wexpand(tmp2,a->top*4);
                bn_sqr_recursive(tmp->d,a->d,a->top,tmp2->d);
                tmp->top=a->top*2;
-               if (tmp->d[tmp->top-1] == 0)
+               if (tmp->top > 0 && tmp->d[tmp->top-1] == 0)
                        tmp->top--;
 #else
                if (!BN_sqr(tmp,a,ctx)) goto err;