SELinux: fix memory leak in security_netlbl_cache_add()

Fix memory leak in security_netlbl_cache_add()
Note: The Coverity checker gets credit for spotting this one.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: James Morris <jmorris@namei.org>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>