scsi: iscsi: set auth_protocol back to NULL if CHAP_A value is not supported

If the CHAP_A value is not supported, the chap_server_open() function
should free the auth_protocol pointer and set it to NULL, or we will leave
a dangling pointer around.

[ 66.010905] Unsupported CHAP_A value
[ 66.011660] Security negotiation failed.
[ 66.012443] iSCSI Login negotiation failed.
[ 68.413924] general protection fault: 0000 [#1] SMP PTI
[ 68.414962] CPU: 0 PID: 1562 Comm: targetcli Kdump: loaded Not tainted 4.18.0-80.el8.x86_64 #1
[ 68.416589] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[ 68.417677] RIP: 0010:__kmalloc_track_caller+0xc2/0x210

Signed-off-by: Maurizio Lombardi <[e-mail]>
Reviewed-by: Chris Leech <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/iscsi/iscsi_target_auth.c

···
 	return CHAP_DIGEST_UNKNOWN;
 }
 
+static void chap_close(struct iscsi_conn *conn)
+{
+	kfree(conn->auth_protocol);
+	conn->auth_protocol = NULL;
+}
+
 static struct iscsi_chap *chap_server_open(
 	struct iscsi_conn *conn,
 	struct iscsi_node_auth *auth,
···
 	case CHAP_DIGEST_UNKNOWN:
 	default:
 		pr_err("Unsupported CHAP_A value\n");
-		kfree(conn->auth_protocol);
+		chap_close(conn);
 		return NULL;
 	}
 
···
 	 * Generate Challenge.
 	 */
 	if (chap_gen_challenge(conn, 1, aic_str, aic_len) < 0) {
-		kfree(conn->auth_protocol);
+		chap_close(conn);
 		return NULL;
 	}
 
 	return chap;
-}
-
-static void chap_close(struct iscsi_conn *conn)
-{
-	kfree(conn->auth_protocol);
-	conn->auth_protocol = NULL;
 }
 
 static int chap_server_compute_md5(