HPCC-8535 Fix cast of a string constant to very large string type

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #3827 from ghalliday/issue8535

HPCC-8535 Fix cast of a very long string constant

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>