cache: make sure the user has specified a callback

In cache_include(), the user callback is used even though one might not
have been specified, leading to a crash. This happens, for example, when
NetworkManager calls nl_cache_include() with the change_cb argument set
to NULL. Fix it by making sure the callback points to a valid address.

Signed-off-by: Mihai Dontu <mihai.dontu@gmail.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>