target/iscsi: avoid NULL dereference in CHAP auth error path

If chap_server_compute_md5() fails early, e.g. via CHAP_N mismatch, then
crypto_free_shash() is called with a NULL pointer which gets
dereferenced in crypto_shash_tfm().

Fixes: 69110e3c ("iscsi-target: Use shash and ahash")
Suggested-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Cc: stable@vger.kernel.org # 4.6+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_auth.c

@@ -421,6 +421,7 @@ static int chap_server_compute_md5(
 	auth_ret = 0;
 out:
 	kzfree(desc);
+	if (tfm)
 		crypto_free_shash(tfm);
 	kfree(challenge);
 	kfree(challenge_binhex);