CVE-2024-33600: nscd: Do not send missing not-found response in addgetnetgrentX (bug 31678)

If we failed to add a not-found response to the cache, the dataset
point can be null, resulting in a null pointer dereference.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 7835b00d)