Require that MAC label buffers be able to store a non-empty string.

The buffer size may be used to initialize an sbuf in
MAC_POLICY_EXTERNALIZE, and without this constraint it's possible to
trigger an assertion failure in the sbuf code.  With INVARIANTS
disabled, the first attempt to write to the sbuf will fail.

Reported by:	pho
Reviewed by:	delphij
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D16527

sys/security/mac/mac_framework.c

@@ -586,7 +586,9 @@ int
 mac_check_structmac_consistent(struct mac *mac)
 {
 
-	if (mac->m_buflen > MAC_MAX_LABEL_BUF_LEN)
+	/* Require that labels have a non-zero length. */
+	if (mac->m_buflen > MAC_MAX_LABEL_BUF_LEN ||
+	    mac->m_buflen <= sizeof(""))
 		return (EINVAL);
 
 	return (0);