HPCC-12645 Don't generate internal errors looking at old workunits

The "kb" stats kind was only used in 5.0.4 to record the peak memory used
by a workunit.  This allows those workunits to be read, but sizes will be
out by a factor of 1000.  Not worth providing full support since it
hasn't been used for anything afaik.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>