Fix NULL pointer dereference in cn_filter()

Fix NULL pointer dereference in cn_filter()

Check that sk_user_data is not NULL, else return from cn_filter().
Could not reproduce this issue, but Oliver Sang verified it has fixed
the "Closes" problem below.

Fixes: 2aa1f7a1f47c ("connector/cn_proc: Add filtering to fix some bugs")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202309201456.84c19e27-oliver.sang@intel.com/
Signed-off-by: Anjali Kulkarni <anjali.k.kulkarni@oracle.com>
Link: https://lore.kernel.org/r/20231020234058.2232347-1-anjali.k.kulkarni@oracle.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>