ovl: fix NULL pointer dereference in copy up warning

This patch is fixing a NULL pointer dereference to get a recently
introduced warning message working.

Fixes: 5b0a414d

 ("ovl: fix filattr copy-up failure")
Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Cc: <stable@vger.kernel.org> # v5.15
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>