HPCC-11865 Clean up string handling in XMlTokenizer when exceptions thrown

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
HPCC-11865 Update based on review

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #6257 from afishbeck/xmlTokenizerClang

HPCC-11865 Clean up string handling in XMlTokenizer when exceptions thrown

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>