Initialize oldvtix

Initialize oldvtix

Initialize the variable to avoid compiler complaints that it
could be used uninitialized.  Use a sentinel value and issue
a warning if we try to use the variable and we haven't set
it to a useful value.

Change-Id: I037be2ad8e0b55d06b50aafdeca5012764640f16
Reviewed-on: http://gerrit.openafs.org/1810
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>