vol: Don't return an uninited val from LookupNode

vol: Don't return an uninited val from LookupNode

If the SalvageHashTable for a particular node is empty, then LookupNode
can return an uninitialised value to the caller. It isn't clear from a
broader code inspection whether LookupNode can ever be called with an
empty SalvageHashTable, but returning a NULL vsp does seem like the
correct thing to do in this situation.

Caught by clang-analyzer

Change-Id: Ibee79e6acf7d3bdbc2995ef924338ade4de77d08
Reviewed-on: http://gerrit.openafs.org/7094
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>