OPENAFS-SA-2018-002 ubik: prevent VOTE_Debug, VOTE_XDebug information leak

OPENAFS-SA-2018-002 ubik: prevent VOTE_Debug, VOTE_XDebug information leak

VOTE_Debug and VOTE_XDebug (udebug) both leave a single field
uninitialized if there is no current transaction.  This leaks the memory
contents of the ubik server over the wire.

struct ubik_debug
- 4 bytes in member writeTrans

In common code to both RPCs, ensure that writeTrans is always
initialized.

[kaduk@mit.edu: switch to memset]

(cherry picked from commit 7a7c1f751cdb06c0d95339c999b2c035c2d2168b)

Change-Id: I2759989bf1a5190f9f03621218224c47094a88b7