kdb: don't dereference a null pointer on corrupt database

kdb: don't dereference a null pointer on corrupt database

When iterating through the database, kdb would dereference a
null pointer if it encountered an error retrieving the value
or if the value was not the right length, in code that was clearly
cut-and-pasted from the other branch of an "if" statement where a
specific entry was requested on the command line.  Print the name
of the entry with the problem as was apparently intended.

Change-Id: Idc2d3c9b6049e5d3b0eb302353a68bcfdad6a90d
Found-by: clang static analyzer
Reviewed-on: http://gerrit.openafs.org/5174
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>