HPCC-12047 String repad causes seg fault on -ve lengths

Technically, this fix is incorrect, as someone COULD be trying to deal with
strings > 2Gb in length.

In practice, anyone trying to deal with strings that size will hit other
problems, and this can be fixed when those are.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-12047 String repad causes seg fault on -ve lengths

Additional check for invalid parameter

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6332 from richardkchapman/repad-negative

HPCC-12047 String repad causes seg fault on -ve lengths

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>