libselinux: align SELABEL_OPT_DIGEST usage with man page

According to selabel_opn(3) a non-null value for this option enables the
generation of an SHA1 digest of the spec files loaded as described in
selabel_digest(3).

Signed-off-by: Christian Göttsche <[email protected]>
Acked-by: James Carter <[email protected]>

libselinux/src/label.c

@@ -62,7 +62,7 @@ static inline struct selabel_digest *selabel_is_digest_set
 
  while (n--) {
  if (opts[n].type == SELABEL_OPT_DIGEST &&
- opts[n].value == (char *)1) {
+ !!opts[n].value) {
  digest = calloc(1, sizeof(*digest));
  if (!digest)
  goto err;