sysctl: fix null checking in bin_dn_node_address()

The null check of `strchr() + 1' is broken, which is always non-null,
leading to OOB read.  Instead, check the result of strchr().

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>