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 <mlombard@redhat.com>
Reviewed-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>