HPCC-8870 Fix a minor possibility of corrupting memory in csv unquoting

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4017 from ghalliday/issue8870

HPCC-8870 Fix a minor possibility of corrupting memory in csv unquoting

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>