vol: nuke() doesn't need to zero code

We don't need to zero the result code two lines before we assign
a new value to it - just remove the pointless assignment

Caught by clang-analyzer

Change-Id: I8fefdc839ef9980d7f7703add3b11b089d524d38
Reviewed-on: http://gerrit.openafs.org/9167
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>