HPCC-10560 Fix incorrect use of utf8CharLen

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5226 from ghalliday/issue10560

HPCC-10560 Fix incorrect use of utf8CharLen

Reviewed-By: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>