appletalk: Fix OOPS in atalk_release().

Commit 60d9f461 ("appletalk: remove
the BKL") added a dereference of "sk" before checking for NULL in
atalk_release().

Guard the code block completely, rather than partially, with the
NULL check.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>