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)

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>
(cherry picked from commit e191cb889c941848fe09f04c24d969111e6dc494)

Change-Id: I517b5772241314a0854094655e25f3f75eba90d1
Reviewed-on: http://gerrit.openafs.org/9358
Tested-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>