volser: Remove needless initialisation

Remove a number of places where we initialise a variable to NULL
immediately before we assign a real value to it.

Caught by clang-analyzer

Change-Id: I74ece8f3a82a5c768d769b74b4ccce12195fc216
Reviewed-on: http://gerrit.openafs.org/9169
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>