usb-storage: fix possible memory leak and missing error message When scsi_bus_legacy_add_drive() return NULL, meanwhile err will be not NULL, which will casue memory leak and missing error message. Cc: Markus Armbruster <armbru@redhat.com> Signed-off-by: Gonglei <arei.gonglei@huawei.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>