viced: Don't write out garbage when extending file

When we extend a file, we write a single byte of data to it - tlen
is used as a source for this single byte. However, in the current
code, tlen is used uninitialised. Set it to 0, so we don't write a
byte of stack garbage.

Caught by coverity (#986015)

Change-Id: I11480a5ba0a5544437e18a6b314ff98ba6256df0
Reviewed-on: http://gerrit.openafs.org/9271
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>