afsdump_extract: clarify logic to avoid freeing local buffer

afsdump_extract: clarify logic to avoid freeing local buffer

Sometimes vnodepath is set to a local buffer.  Sometimes it is set
to malloc'ed storage.  Simplify the logic for freeing vnodepath
by checking explicitly for this condition rather than the state
of other variables.  As a bonus, avoids a false (?) positive from
the static analyzer.

Change-Id: I3772cb97698acc5a6ac1f438977c673e6fea7722
Reviewed-on: http://gerrit.openafs.org/7869
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>