smb311_decode_neg_context() doesn't properly check against SMB packet
boundaries prior to accessing individual negotiate context entries. This
is due to the length check omitting the eight byte smb2_neg_context
header, as well as incorrect decrementing of len_of_ctxts.
Fixes: 5100d8a3fe03 ("SMB311: Improve checking of negotiate security contexts") Reported-by: Volker Lendecke <vl@samba.org> Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com> Signed-off-by: David Disseldorp <ddiss@suse.de> Signed-off-by: Steve French <stfrench@microsoft.com>