scsi/lsi53c895a: fix use-after-free in lsi_do_msgout (CVE-2022-0216)

scsi/lsi53c895a: fix use-after-free in lsi_do_msgout (CVE-2022-0216)

Set current_req->req to NULL to prevent reusing a free'd buffer in case of
repeated SCSI cancel requests. Thanks to Thomas Huth for suggesting the patch.

Fixes: CVE-2022-0216
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/972
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220705200543.2366809-1-mcascell@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>