HPCC-12762 Multi filenames using wildcards allocate large amounts of memory

Previous bug was hiding a second bug that caused prefix not to be applied.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6777 from richardkchapman/multilfn-bug2

HPCC-12762 Multi filenames using wildcards allocate large amounts of memory

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>