OPENAFS-SA-2018-002 butc: Initialize OUT scalar value

OPENAFS-SA-2018-002 butc: Initialize OUT scalar value

In STC_ReadLabel, the interaction with the tape device is
synchronous, so there is no need to allocate a task ID for status
monitoring.  However, we do need to initialize the output value,
to avoid writing stack garbage on the wire.

(cherry picked from commit f5a80115f8f7f9418287547f0fc7fdb13d936f00)

(cherry picked from commit 418b2ab56c60e44375df31a3a8f77461d577a5ff)

(cherry picked from commit babbb2824a5e3d6210b9079ab08f8771ac6ef892)

Change-Id: Ie18bbe7542a23d2ce952cfcd5288ee0aa43bb71f