smackfs: Prevent underflow in smk_set_cipso()

There is a upper bound to "catlen" but no lower bound to prevent
negatives.  I don't see that this necessarily causes a problem but we
may as well be safe.

Fixes: e114e473 ("Smack: Simplified Mandatory Access Control Kernel")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smackfs.c

@@ -896,7 +896,7 @@ static ssize_t smk_set_cipso(struct file *file, const char __user *buf,
 	}
 
 	ret = sscanf(rule, "%d", &catlen);
-	if (ret != 1 || catlen > SMACK_CIPSO_MAXCATNUM)
+	if (ret != 1 || catlen < 0 || catlen > SMACK_CIPSO_MAXCATNUM)
 		goto out;
 
 	if (format == SMK_FIXED24_FMT &&