Avoid creating illegal pointers

Found by tis-interpreter
Reviewed-by: NRich Salz <rsalz@openssl.org>

GH: #1179

crypto/bn/bn_lib.c

@@ -565,9 +565,9 @@ BIGNUM *BN_lebin2bn(const unsigned char *s, int len, BIGNUM *ret)
     if (ret == NULL)
         return (NULL);
     bn_check_top(ret);
-    s += len - 1;
+    s += len;
     /* Skip trailing zeroes. */
-    for ( ; len > 0 && *s == 0; s--, len--)
+    for ( ; len > 0 && s[-1] == 0; s--, len--)
         continue;
     n = len;
     if (n == 0) {
@@ -584,7 +584,8 @@ BIGNUM *BN_lebin2bn(const unsigned char *s, int len, BIGNUM *ret)
     ret->neg = 0;
     l = 0;
     while (n--) {
-        l = (l << 8L) | *(s--);
+        s--;
+        l = (l << 8L) | *s;
         if (m-- == 0) {
             ret->d[--i] = l;
             l = 0;
@@ -610,10 +611,11 @@ int BN_bn2lebinpad(const BIGNUM *a, unsigned char *to, int tolen)
     /* Add trailing zeroes if necessary */
     if (tolen > i)
         memset(to + i, 0, tolen - i);
-    to += i - 1;
+    to += i;
     while (i--) {
         l = a->d[i / BN_BYTES];
-        *(to--) = (unsigned char)(l >> (8 * (i % BN_BYTES))) & 0xff;
+        to--;
+        *to = (unsigned char)(l >> (8 * (i % BN_BYTES))) & 0xff;
     }
     return tolen;
 }