Don't let top go below zero!

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;