audit: introduce audit_session_is_valid() and make use of it everywhere

Let's add a proper validation function, since validation isn't entirely
trivial. Make use of it where applicable. Also make use of
AUDIT_SESSION_INVALID where we need a marker for an invalid audit
session.

src/basic/audit-util.h

@@ -29,3 +29,7 @@ int audit_session_from_pid(pid_t pid, uint32_t *id);
 int audit_loginuid_from_pid(pid_t pid, uid_t *uid);
 
 bool use_audit(void);
+
+static inline bool audit_session_is_valid(uint32_t id) {
+        return id > 0 && id != AUDIT_SESSION_INVALID;
+}